root@wandboard:~/rubygems-2.6.12# gem install mmap
Building native extensions.  This could take a while...
ERROR:  Error installing mmap:
	ERROR: Failed to build gem native extension.

    current directory: /usr/lib/ruby/gems/2.2.0/gems/mmap-0.2.6/ext/mmap
/usr/bin/ruby -r ./siteconf20170519-839-1jqndj1.rb extconf.rb
creating Makefile

current directory: /usr/lib/ruby/gems/2.2.0/gems/mmap-0.2.6/ext/mmap
make "DESTDIR=" clean

current directory: /usr/lib/ruby/gems/2.2.0/gems/mmap-0.2.6/ext/mmap
make "DESTDIR="
compiling mmap.c
In file included from /usr/include/ruby-2.2.0/ruby/ruby.h:29:0,
                 from /usr/include/ruby-2.2.0/ruby.h:33,
                 from mmap.c:1:
/usr/include/ruby-2.2.0/ruby/defines.h:26:19: fatal error: stdio.h: No such file or directory
compilation terminated.
Makefile:237: recipe for target 'mmap.o' failed
make: *** [mmap.o] Error 1

make failed, exit code 2

Gem files will remain installed in /usr/lib/ruby/gems/2.2.0/gems/mmap-0.2.6 for inspection.
Results logged to /usr/lib/ruby/gems/2.2.0/extensions/arm-linux/2.2.0/mmap-0.2.6/gem_make.out