QPID-4495: RVM fails to use C++ compiler for the Ruby gem.

mkmf tries to find a basic working compiler and linker first, and when
it finds gcc it stops. But since Qpid is written in C++ this causes the
gem to fail to install when the C++ libraries checked.

This fix forces mkmf from the outset to use a C++ compiler instead.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1418488 13f79535-47bb-0310-9956-ffa450edef68