Improve test for g++ 4.6
Skip the g++ 4.6 test if we're not going to build any C++ source.
If a test has C++ source files we automatically determine which C++
compiler to use based on $CC (for example, clang++ if CC=clang).
However, this is not done for tests without C++ source and CXX will
be GNU make's default of g++. This produces suprious "g++: not found"
errors in testrun output on systems without a gcc/g++.
Differential Revision: http://reviews.llvm.org/D10122