predatorian@reincarnate:~/Downloads/ruby-2.3.1
$ RBENV_VERSION=2.3.1 gem install tk
Building native extensions.  This could take a while...
ERROR:  Error installing tk:
	ERROR: Failed to build gem native extension.

    current directory: /home/predatorian/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tk-0.1.1/ext/tk
/home/predatorian/.rbenv/versions/2.3.1/bin/ruby -r ./siteconf20160925-4025-nw2gr5.rb extconf.rb
check functions.checking for ruby_native_thread_p() in ruby.h... yes
checking for rb_errinfo() in ruby.h... yes
checking for rb_safe_level() in ruby.h... yes
checking for rb_hash_lookup() in ruby.h... yes
checking for rb_proc_new() in ruby.h... yes
checking for rb_obj_untrust() in ruby.h... yes
checking for rb_obj_taint() in ruby.h... yes
checking for rb_set_safe_level_force() in ruby.h... yes
checking for rb_sourcefile() in ruby.h... yes
checking for rb_thread_alive_p() in ruby.h... no
checking for rb_thread_check_trap_pending() in ruby.h... yes
checking for ruby_enc_find_basename() in ruby.h... yes
check libraries.checking for t_open() in -lnsl... no
checking for socket() in -lsocket... no
checking for dlopen() in -ldl... yes
checking for log() in -lm... yes
Use ActiveTcl libraries (if available).
Search tclConfig.sh and tkConfig.sh.
Fail to find [tclConfig.sh, tkConfig.sh]
Use X11 libraries (or use TK_XINCLUDES/TK_XLIBSW information on tkConfig.sh).
checking for XOpenDisplay() in -lX11... yes
Search tcl.h
checking for tcl.h... no
Search tk.h
checking for tk.h... no
Search Tcl library...........Search Tcl library...........*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/home/predatorian/.rbenv/versions/2.3.1/bin/$(RUBY_BASE_NAME)
	--enable-shared
	--disable-shared
	--with-tk-old-extconf
	--without-tk-old-extconf
	--with-tk-old-extconf
	--without-tk-old-extconf
	--with-ActiveTcl
	--without-ActiveTcl
	--with-tk-shlib-search-path
	--without-tk-shlib-search-path
	--with-tcltkversion
	--without-tcltkversion
	--with-tcl-build-dir
	--without-tcl-build-dir
	--with-tk-build-dir
	--without-tk-build-dir
	--with-tcl-config
	--without-tcl-config
	--with-tk-config
	--without-tk-config
	--with-tclConfig-dir
	--without-tclConfig-dir
	--with-tkConfig-dir
	--without-tkConfig-dir
	--with-tclConfig-file
	--without-tclConfig-file
	--with-tkConfig-file
	--without-tkConfig-file
	--with-tcllib
	--without-tcllib
	--with-tklib
	--without-tklib
	--with-tcl-dir
	--without-tcl-dir
	--with-tk-dir
	--without-tk-dir
	--with-tcl-include
	--without-tcl-include
	--with-tk-include
	--without-tk-include
	--with-tcl-lib
	--without-tcl-lib
	--with-tk-lib
	--without-tk-lib
	--with-tcltk-framework
	--without-tcltk-framework
	--with-tcl-framework-dir
	--without-tcl-framework-dir
	--with-tk-framework-dir
	--without-tk-framework-dir
	--with-tcl-framework-header
	--without-tcl-framework-header
	--with-tk-framework-header
	--without-tk-framework-header
	--with-X11
	--without-X11
	--with-X11-dir
	--without-X11-dir
	--with-X11-include
	--without-X11-include
	--with-X11-lib
	--without-X11-lib
	--enable-tcltk-stubs
	--disable-tcltk-stubs
	--enable-tcl-h-ver-check
	--disable-tcl-h-ver-check
	--enable-tk-h-ver-check
	--disable-tk-h-ver-check
	--enable-mac-tcltk-framework
	--disable-mac-tcltk-framework
	--enable-tcltk-framework
	--disable-tcltk-framework
	--enable-pthread
	--disable-pthread
	--enable-tcl-thread
	--disable-tcl-thread
	--enable-space-on-tk-libpath
	--disable-space-on-tk-libpath
	--with-nsllib
	--without-nsllib
	--with-socketlib
	--without-socketlib
	--with-dllib
	--without-dllib
	--with-mlib
	--without-mlib
	--with-tcl-build-dir
	--without-tcl-build-dir
	--with-tk-build-dir
	--without-tk-build-dir
	--with-tcltkversion
	--without-tcltkversion
	--with-ActiveTcl
	--without-ActiveTcl
	--enable-space-on-tk-libpath
	--disable-space-on-tk-libpath
	--enable-tcltk-stubs
	--disable-tcltk-stubs
	--with-tcltk-stubs
	--without-tcltk-stubs
	--with-tcl-dir
	--without-tcl-dir
	--with-tcl-include
	--without-tcl-include=${tcl-dir}/include
	--with-tcl-lib
	--without-tcl-lib=${tcl-dir}/lib
	--with-tk-dir
	--without-tk-dir
	--with-tk-include
	--without-tk-include=${tk-dir}/include
	--with-tk-lib
	--without-tk-lib=${tk-dir}/lib
	--with-tclConfig-file
	--without-tclConfig-file
	--with-tkConfig-file
	--without-tkConfig-file
	--with-tclConfig-dir
	--without-tclConfig-dir
	--with-tkConfig-dir
	--without-tkConfig-dir
	--with-tcl-framework-header
	--without-tcl-framework-header
	--with-tk-framework-header
	--without-tk-framework-header
	--with-tcl-framework-dir
	--without-tcl-framework-dir
	--with-tk-framework-dir
	--without-tk-framework-dir
	--with-tcltk-framework
	--without-tcltk-framework
	--enable-tcltk-framework
	--disable-tcltk-framework
	--with-tcltk-framework
	--without-tcltk-framework
	--enable-tcltk-framework
	--disable-tcltk-framework
	--enable-mac-tcltk-framework
	--disable-mac-tcltk-framework
	--with-tk-shlib-search-path
	--without-tk-shlib-search-path
	--with-tklib
	--without-tklib
	--with-tcllib
	--without-tcllib
	--with-X11
	--without-X11
	--with-X11-dir
	--without-X11-dir
	--with-X11-include
	--without-X11-include=${X11-dir}/include
	--with-X11-lib
	--without-X11-lib=${X11-dir}/lib
	--with-X11-lib
	--without-X11-lib
	--with-X11lib
	--without-X11lib
	--enable-tcl-h-ver-check
	--disable-tcl-h-ver-check
	--enable-tk-h-ver-check
	--disable-tk-h-ver-check

Warning:: cannot find Tcl library. tcltklib will not be compiled (tcltklib is disabled on your Ruby. That is, Ruby/Tk will not work). Please check configure options.

Can't find proper Tcl/Tk libraries. So, can't make tcltklib.so which is required by Ruby/Tk.
If you have Tcl/Tk libraries on your environment, you may be able to use them with configure options (see ext/tk/README.tcltklib).
At present, Tcl/Tk8.6 is not supported. Although you can try to use Tcl/Tk8.6 with configure options, it will not work correctly. I recommend you to use Tcl/Tk8.5 or 8.4.

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /home/predatorian/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-linux/2.3.0-static/tk-0.1.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /home/predatorian/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tk-0.1.1 for inspection.
Results logged to /home/predatorian/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-linux/2.3.0-static/tk-0.1.1/gem_make.out