Objective C/C++: Honor CC and CXX env vars to select compiler

If the `OBJC` or `OBJCXX` environment variable is not set to specify an
Objective C or C++ compiler, check `CC` or `CXX` too.

Fixes: #20703

5 files changed, 37 insertions, 18 deletions

diff --git a/Help/envvar/OBJC.rst b/Help/envvar/OBJC.rst
index dda809c8e6..30c0d133b9 100644
--- a/Help/envvar/OBJC.rst
+++ b/Help/envvar/OBJC.rst
@@ -9,3 +9,6 @@ which the value for ``OBJC`` is stored in the cache as
 :variable:`CMAKE_OBJC_COMPILER <CMAKE_<LANG>_COMPILER>`. For any configuration
 run (including the first), the environment variable will be ignored if the
 :variable:`CMAKE_OBJC_COMPILER <CMAKE_<LANG>_COMPILER>` variable is defined.
+
+If ``OBJC`` is not defined, the :envvar:`CC` environment variable will
+be checked instead.
diff --git a/Help/envvar/OBJCXX.rst b/Help/envvar/OBJCXX.rst
index be1fa4248c..a72f7e7104 100644
--- a/Help/envvar/OBJCXX.rst
+++ b/Help/envvar/OBJCXX.rst
@@ -9,3 +9,6 @@ which the value for ``OBJCXX`` is stored in the cache as
 :variable:`CMAKE_OBJCXX_COMPILER <CMAKE_<LANG>_COMPILER>`. For any configuration
 run (including the first), the environment variable will be ignored if the
 :variable:`CMAKE_OBJCXX_COMPILER <CMAKE_<LANG>_COMPILER>` variable is defined.
+
+If ``OBJCXX`` is not defined, the :envvar:`CXX` environment variable will
+be checked instead.
diff --git a/Help/release/3.16.rst b/Help/release/3.16.rst
index e2d6788217..bd6d041f5f 100644
--- a/Help/release/3.16.rst
+++ b/Help/release/3.16.rst
@@ -303,3 +303,10 @@ Changes made since CMake 3.16.0 include the following.
   Additionally, the modules no longer expose their internal ``_Python*``
   cache entries publicly.  CMake 3.16.0 through 3.16.4 accidentally
   made them visible as advanced cache entries.
+
+3.16.7
+------
+
+* Selection of the Objective C or C++ compiler now considers the
+  :envvar:`CC` or :envvar:`CXX` environment variable if the
+  :envvar:`OBJC` or :envvar:`OBJCXX` environment variable is not set.
diff --git a/Modules/CMakeDetermineOBJCCompiler.cmake b/Modules/CMakeDetermineOBJCCompiler.cmake
index ad13eab5e8..11b47fde84 100644
--- a/Modules/CMakeDetermineOBJCCompiler.cmake
+++ b/Modules/CMakeDetermineOBJCCompiler.cmake
@@ -34,16 +34,19 @@ else()
   if(NOT CMAKE_OBJC_COMPILER)
     set(CMAKE_OBJC_COMPILER_INIT NOTFOUND)
 
-    # prefer the environment variable OBJC
-    if($ENV{OBJC} MATCHES ".+")
-      get_filename_component(CMAKE_OBJC_COMPILER_INIT $ENV{OBJC} PROGRAM PROGRAM_ARGS CMAKE_OBJC_FLAGS_ENV_INIT)
-      if(CMAKE_OBJC_FLAGS_ENV_INIT)
-        set(CMAKE_OBJC_COMPILER_ARG1 "${CMAKE_OBJC_FLAGS_ENV_INIT}" CACHE STRING "First argument to Objective-C compiler")
+    # prefer the environment variable OBJC or CC
+    foreach(var OBJC CC)
+      if($ENV{${var}} MATCHES ".+")
+        get_filename_component(CMAKE_OBJC_COMPILER_INIT $ENV{${var}} PROGRAM PROGRAM_ARGS CMAKE_OBJC_FLAGS_ENV_INIT)
+        if(CMAKE_OBJC_FLAGS_ENV_INIT)
+          set(CMAKE_OBJC_COMPILER_ARG1 "${CMAKE_OBJC_FLAGS_ENV_INIT}" CACHE STRING "First argument to Objective-C compiler")
+        endif()
+        if(NOT EXISTS ${CMAKE_OBJC_COMPILER_INIT})
+          message(FATAL_ERROR "Could not find compiler set in environment variable ${var}:\n  $ENV{${var}}")
+        endif()
+        break()
       endif()
-      if(NOT EXISTS ${CMAKE_OBJC_COMPILER_INIT})
-        message(FATAL_ERROR "Could not find compiler set in environment variable OBJC:\n$ENV{OBJC}.")
-      endif()
-    endif()
+    endforeach()
 
     # next try prefer the compiler specified by the generator
     if(CMAKE_GENERATOR_OBJC)
diff --git a/Modules/CMakeDetermineOBJCXXCompiler.cmake b/Modules/CMakeDetermineOBJCXXCompiler.cmake
index 60fcbb3f51..db874d1a68 100644
--- a/Modules/CMakeDetermineOBJCXXCompiler.cmake
+++ b/Modules/CMakeDetermineOBJCXXCompiler.cmake
@@ -36,16 +36,19 @@ else()
   if(NOT CMAKE_OBJCXX_COMPILER)
     set(CMAKE_OBJCXX_COMPILER_INIT NOTFOUND)
 
-    # prefer the environment variable OBJCXX
-    if($ENV{OBJCXX} MATCHES ".+")
-      get_filename_component(CMAKE_OBJCXX_COMPILER_INIT $ENV{OBJCXX} PROGRAM PROGRAM_ARGS CMAKE_OBJCXX_FLAGS_ENV_INIT)
-      if(CMAKE_OBJCXX_FLAGS_ENV_INIT)
-        set(CMAKE_OBJCXX_COMPILER_ARG1 "${CMAKE_OBJCXX_FLAGS_ENV_INIT}" CACHE STRING "First argument to Objective-C++ compiler")
+    # prefer the environment variable OBJCXX or CXX
+    foreach(var OBJCXX CXX)
+      if($ENV{${var}} MATCHES ".+")
+        get_filename_component(CMAKE_OBJCXX_COMPILER_INIT $ENV{${var}} PROGRAM PROGRAM_ARGS CMAKE_OBJCXX_FLAGS_ENV_INIT)
+        if(CMAKE_OBJCXX_FLAGS_ENV_INIT)
+          set(CMAKE_OBJCXX_COMPILER_ARG1 "${CMAKE_OBJCXX_FLAGS_ENV_INIT}" CACHE STRING "First argument to Objective-C++ compiler")
+        endif()
+        if(NOT EXISTS ${CMAKE_OBJCXX_COMPILER_INIT})
+          message(FATAL_ERROR "Could not find compiler set in environment variable ${var}:\n  $ENV{${var}}")
+        endif()
+        break()
       endif()
-      if(NOT EXISTS ${CMAKE_OBJCXX_COMPILER_INIT})
-        message(FATAL_ERROR "Could not find compiler set in environment variable OBJCXX:\n$ENV{OBJCXX}.\n${CMAKE_OBJCXX_COMPILER_INIT}")
-      endif()
-    endif()
+    endforeach()
 
     # next prefer the generator specified compiler
     if(CMAKE_GENERATOR_OBJCXX)