$ sudo gem install mysql -- --with-mysql-dir/Applications/XAMPP/xamppfiles
Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
	ERROR: Failed to build gem native extension.

/usr/local/bin/ruby extconf.rb --with-mysql-dir/Applications/XAMPP/xamppfiles
extconf.rb:4: warning: Insecure world writable dir /usr/local in PATH, mode 040757
extconf.rb:4: warning: Insecure world writable dir /usr/local in PATH, mode 040757
extconf.rb:7: warning: Insecure world writable dir /usr/local in PATH, mode 040757
extconf.rb:17: warning: Insecure world writable dir /usr/local in PATH, mode 040757
checking for mysql_ssl_set()... yes
checking for rb_str_set_len()... no
checking for rb_thread_start_timer()... no
checking for mysql.h... no
checking for mysql/mysql.h... no
*** 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=/usr/local/bin/ruby
	--with-mysql-config
	--without-mysql-config