Howto get include directories (ruby.h) for Linux Ruby C extension project?

If you were using autoconf, you could borrow ruby.ac from rice:

http://github.com/jameskilton/rice/blob/master/ruby.ac

or since you are using a different build system, you can duplicate its behavior. To summarize what it does:

1. We use the rbconfig module to get ruby config variables like this:

   $RUBY -rrbconfig -e "puts(Config::CONFIG['$1'] || '')" <variable>

   where $RUBY is the ruby interpreter (sometimes interpreters get installed with a different name, e.g. ruby1.8 or ruby1.9) and <variable> is the desired config variable
2. We then set our build variables:

   
     if ${ruby_config_rubyhdrdir} is empty: (e.g. ruby 1.8)
       CPPFLAGS="-I${ruby_config_archdir}
     else:
       CPPFLAGS="-I${ruby_config_rubyhdrdir} ${ruby_config_rubyhdrdir}/${arch}
   
     CFLAGS="${ruby_config_CFLAGS} ${ruby_config_CCDLFLAGS}"
   
     LDFLAGS="-L${ruby_config_archdir} -L${ruby_config_libdir} ${ruby_config_LDFLAGS}"
   
     LIBS="${ruby_config_LIBS} ${ruby_config_DLDLIBS}"
   

   where the variables ${ruby_config_*} are determined using the config command above, e.g.:

   ruby_config_foo=$RUBY -rrbconfig -e "puts(Config::CONFIG['$1'] || '')" foo

3. We also need to link with the ruby library, so we append ${ruby_config_LIBRUBYARG}.
4. The above variables are for compiling and linking; you may also need to install. Ruby library files should be installed to ${ruby_config_sitelibdir}. Ruby extensions should be installed to ${ruby_config_sitearchdir}.
5. There's also some magic in ruby.ac for building with mingw and linking against the one-click installer on windows (which used to be built with vc6; I'm not sure if it still is or not).