Merge topic 'Apple-compiler-selection'

da928d30 Help: Add notes for topic 'Apple-compiler-selection'
1f085e11 OS X: Resolve compiler in /usr/bin to that reported by Xcode xcrun
85d31735 CMakeDetermineCompiler: Factor out xcrun invocation into a macro

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

@@ -0,0 +1,8 @@
+Apple-compiler-selection
+------------------------
+
+* On OS X with Makefile and Ninja generators, when a compiler is found
+  in ``/usr/bin`` it is now mapped to the corresponding compiler inside
+  the Xcode application folder, if any.  This allows such build
+  trees to continue to work with their original compiler even when
+  ``xcode-select`` switches to a different Xcode installation.

Modules/CMakeDetermineCompiler.cmake

@@ -71,17 +71,32 @@ macro(_cmake_find_compiler lang)
   unset(_languages)
 
   # Look for a make tool provided by Xcode
-  if(CMAKE_${lang}_COMPILER STREQUAL "CMAKE_${lang}_COMPILER-NOTFOUND" AND CMAKE_HOST_APPLE)
-    foreach(comp ${CMAKE_${lang}_COMPILER_LIST})
-      execute_process(COMMAND xcrun --find ${comp}
+  if(CMAKE_HOST_APPLE)
+    macro(_query_xcrun compiler_name result_var_keyword result_var)
+      if(NOT "x${result_var_keyword}" STREQUAL "xRESULT_VAR")
+        message(FATAL_ERROR "Bad arguments to macro")
+      endif()
+      execute_process(COMMAND xcrun --find ${compiler_name}
         OUTPUT_VARIABLE _xcrun_out OUTPUT_STRIP_TRAILING_WHITESPACE
         ERROR_VARIABLE _xcrun_err)
-      if(_xcrun_out)
-        set_property(CACHE CMAKE_${lang}_COMPILER PROPERTY VALUE "${_xcrun_out}")
+      set("${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 ${CMAKE_${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()
+  endif()
 endmacro()
 
 macro(_cmake_find_compiler_path lang)