Teach modules CMakeDetermineCompiler and CMakeUnixFindMake to ask Xcode
where to find the compiler or make tools, using 'xcrun --find', if none
is found in the PATH. Teach module Platform/Darwin to add the path to
the SDK to CMAKE_SYSTEM_PREFIX_PATH so that find_* command look there.
Also add the SDK /usr/include directory to the implicit include list in
CMAKE_${lang}_IMPLICIT_INCLUDE_DIRECTORIES to suppress explicit -I
options for it.
This adds copyright/license notification blocks CMake's non-find
modules. Most of the modules had no notices at all. Some had notices
referring to the BSD license already. This commit normalizes existing
notices and adds missing notices.