d_do_test: make used C++ compiler configurable

Recent changes to allow compiling C++ files in the test suite hard code the win64 compiler to \"Program Files (x86)"\"Microsoft Visual Studio 10.0"\VC\bin\amd64\cl.exe which doesn't even work with a standard installation of VS2010 if the dmd repository is on another drive.

This patch uses the environment varaible CC to obtain the C++ compiler to use and falls back to the current defaults if not set.