CMakeDetermine*Compiler: Factor out search for compiler in PATH

Factor out a _cmake_find_compiler_path helper macro to avoid duplication
of the search for a full path to the compiler.
Xcode: Convert forced CMAKE_<LANG>_COMPILER to full path if possible

During cross-compiling the toolchain file may use CMakeForceCompiler to
force a compiler setting.  When using the Xcode generator try to convert
it to a full path by searching the PATH as is done for the Makefile
generators.
Merge branch 'fix-xcode-compiler-force' into release
find_dependency: Make sure invalid EXACT use can be reported.

Test the first argument directly for matching 'EXACT'. The error
check in its previous position was incorrect and would only trigger
with a version of '0' or similar.
Merge branch 'fix-find_dependency-EXACT' into release