mkmf: skip if C++ compiler not found

Author: nobu

test/mkmf/test_egrep_cpp.rb

@@ -20,6 +20,8 @@ def test_cxx_egrep_cpp
       #error not C++
       #endif
       SRC
+    rescue Errno::ENOENT
+      omit "C++ compiler not available: #{$!.message}"
     end
   end
 end