Kill /NODEFAULTLIB warnings when compiling tests under MSVC in debug

They were caused by gtest/gmock statically linking to the release
runtime in debug mode.
The fix has required patching gmock/CMakeLists & gtest/CMakeLists to remove
hard-coded /MD switch. This patch has already been incorporated in to
gmock 1.7.0 but we cannot upgrade since this does not compile with the
std=c++0x flag on gcc.
Refs #8408
Make sure gtest doesn't overwrite flags that are already set.

Refs #8408
Merge remote-tracking branch 'origin/bugfix/8408_fix_msvc_linker_warnings'