CTest: Report to CDash the compiler version used to build the project

The compiler version reported since commit v2.8.2~1018 (CTest-side
support for compiler name and compiler version, 2009-10-12) only
reported the version of the compiler used to build CMake, and only
if it defined "_COMPILER_VERSION".  Instead use the version of
the compiler used to build the project CTest is testing.