improve portability of Makefiles

[brendan-simon-indt]

Improve portability of Makefiles by finding the cmake tool in the user's path, rather than hardcoding it. On MSYS2 environment, cmake is located at:

• /ucrt64/bin/cmake (for UCRT64 environment)
• /usr/bin/cmake (for MSYS2 environment)

Improve portability by not hard coding top-level directory path. Instead determine top-level path from the current Makefile.

For test-ng-pp I took the liberty of assuming test-ng was installed alongside mockcpp, made the directory relative to it.