Improve mkmf error message

* lib/mkmf.rb: Improve the error message when ruby.h is missing,
  to suggest installing separate packages.  [Feature #14656]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63061 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -241,7 +241,12 @@ module MakeMakefile
     $topdir ||= RbConfig::CONFIG["topdir"]
     $arch_hdrdir = "$(extout)/include/$(arch)"
   else
-    abort "mkmf.rb can't find header files for ruby at #{$hdrdir}/ruby.h"
+    abort <<MESSAGE
+mkmf.rb can't find header files for ruby at #{$hdrdir}/ruby.h
+
+You might have to install separate package for the ruby development
+environment, ruby-dev or ruby-devel for example.
+MESSAGE
   end
 
   CONFTEST = "conftest".freeze