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 topic 'fix-xcode-compiler-force' into next

8c9bfac3 Xcode: Convert forced CMAKE_<LANG>_COMPILER to full path if possible
6852fb80 CMakeDetermine*Compiler: Factor out search for compiler in PATH