Clarify the libgit2 version requirements.

Author: arthurschreiber

ext/rugged/extconf.rb

@@ -43,7 +43,7 @@ def sys(cmd)
     break if major && minor
   end
 
-  try_compile(<<-SRC) or abort "libgit2 version is not compatible"
+  try_compile(<<-SRC) or abort "libgit2 version is not compatible, expected ~> #{major}.#{minor}.0"
 #include <git2/version.h>
 
 #if LIBGIT2_VER_MAJOR != #{major} || LIBGIT2_VER_MINOR != #{minor}