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.

Modules/CMakeUnixFindMake.cmake

@@ -14,3 +14,13 @@
 
 find_program(CMAKE_MAKE_PROGRAM NAMES gmake make smake)
 mark_as_advanced(CMAKE_MAKE_PROGRAM)
+
+# Look for a make tool provided by Xcode
+if(NOT CMAKE_MAKE_PROGRAM AND CMAKE_HOST_APPLE)
+  execute_process(COMMAND xcrun --find make
+    OUTPUT_VARIABLE _xcrun_out OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_VARIABLE _xcrun_err)
+  if(_xcrun_out)
+    set_property(CACHE CMAKE_MAKE_PROGRAM PROPERTY VALUE "${_xcrun_out}")
+  endif()
+endif()