OS X: Ask Xcode for compiler only if it is NOTFOUND

If the user explicitly sets CMAKE_<LANG>_COMPILER to an empty string
then honor it instead of asking Xcode for a compiler.
OS X: Enable command-line build without tools in PATH

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.
Merge topic 'osx-no-command-line-tools' into next

2a0241a OS X: Ask Xcode for compiler only if it is NOTFOUND
Merge topic 'osx-no-command-line-tools' into next

03ab170 OS X: Enable command-line build without tools in PATH