macOS: Revert "Resolve compiler in /usr/bin to that reported by Xcode xcrun"

Revert commit 1f085e11e4 (OS X: Resolve compiler in /usr/bin to that
reported by Xcode xcrun, 2015-01-03, v3.2.0-rc1~126^2~1).  The compilers
in Xcode cannot be used by build systems that do not add an `-isysroot`
flag.

Issue: #19180

Help/release/dev/apple-compiler-selection.rst

@@ -0,0 +1,13 @@
+apple-compiler-selection
+------------------------
+
+* On macOS with :ref:`Ninja Generators` and :ref:`Makefile Generators`, when
+  a compiler is found in ``/usr/bin``, it is now used as-is and is no longer
+  mapped to the corresponding compiler inside Xcode.  The mapping was
+  introduced by CMake 3.2 to allow build trees to continue to work with their
+  original compiler even when ``xcode-select`` switches to a different
+  Xcode installation.  However, the compilers inside Xcode cannot be used
+  without explicit ``-isysroot`` flags and are therefore not suitable for
+  passing to arbitrary third-party build systems.  Furthermore, the mapping
+  behavior can override user-specified compiler paths.  Therefore, this
+  behavior has been reverted.

Modules/CMakeDetermineCompiler.cmake

@@ -84,7 +84,7 @@ macro(_cmake_find_compiler lang)
   unset(_languages)
 
   # Look for a make tool provided by Xcode
-  if(CMAKE_HOST_APPLE)
+  if(CMAKE_${lang}_COMPILER STREQUAL "CMAKE_${lang}_COMPILER-NOTFOUND" AND CMAKE_HOST_APPLE)
     macro(_query_xcrun compiler_name result_var_keyword result_var)
       if(NOT "x${result_var_keyword}" STREQUAL "xRESULT_VAR")
         message(FATAL_ERROR "Bad arguments to macro")
@@ -95,20 +95,13 @@ macro(_cmake_find_compiler lang)
       set("${result_var}" "${_xcrun_out}")
     endmacro()
 
-    set(xcrun_result)
-    if (CMAKE_${lang}_COMPILER MATCHES "^/usr/bin/(.+)$")
-      _query_xcrun("${CMAKE_MATCH_1}" RESULT_VAR xcrun_result)
-    elseif (CMAKE_${lang}_COMPILER STREQUAL "CMAKE_${lang}_COMPILER-NOTFOUND")
-      foreach(comp IN LISTS CMAKE_${lang}_COMPILER_LIST)
-        _query_xcrun("${comp}" RESULT_VAR xcrun_result)
-        if(xcrun_result)
-          break()
-        endif()
-      endforeach()
-    endif()
-    if (xcrun_result)
-      set_property(CACHE CMAKE_${lang}_COMPILER PROPERTY VALUE "${xcrun_result}")
-    endif()
+    foreach(comp IN LISTS CMAKE_${lang}_COMPILER_LIST)
+      _query_xcrun("${comp}" RESULT_VAR xcrun_result)
+      if(xcrun_result)
+        set_property(CACHE CMAKE_${lang}_COMPILER PROPERTY VALUE "${xcrun_result}")
+        break()
+      endif()
+    endforeach()
   endif()
 endmacro()