It accepted an optional argument to test for equality, but no way
to get the linker language of a particular target.
TARGET_PROPERTY provides this flexibility and STREQUAL provides
the necessary API for equality test.
Extend the CompileDefinitions test to cover accessing the
property of another target.
They can't be used when evaluating link libraries, but they can be
used for include directories and compile definitions. Later they can
be used for compile options.