pythongh-100384: Error on unguarded-availability in macOS builds (python#128155)

Generate a build error on ``unguarded-availability`` in portable macOS builds (i.e. using MACOSX_DEPLOYMENT_TARGET), preventing invalid use of symbols that are not available in older versions of the OS.

Author: zanieb

.github/workflows/reusable-macos.yml

@@ -45,6 +45,7 @@ jobs:
         brew link --overwrite tcl-tk@8
     - name: Configure CPython
       run: |
+        MACOSX_DEPLOYMENT_TARGET=10.15 \
         GDBM_CFLAGS="-I$(brew --prefix gdbm)/include" \
         GDBM_LIBS="-L$(brew --prefix gdbm)/lib -lgdbm" \
         ./configure \

Misc/NEWS.d/next/Build/2024-12-21-09-56-37.gh-issue-100384.Ib-XrN.rst

@@ -0,0 +1,2 @@
+Error on ``unguarded-availability`` in macOS builds, preventing invalid
+use of symbols that are not available in older versions of the OS.

configure

@@ -2603,6 +2603,13 @@ AS_VAR_IF([ac_cv_gcc_compat], [yes], [
         esac
         AC_MSG_RESULT([$CC])
 
+        # Error on unguarded use of new symbols, which will fail at runtime for
+        # users on older versions of macOS
+        AX_CHECK_COMPILE_FLAG([-Wunguarded-availability],
+            [AS_VAR_APPEND([CFLAGS_NODIST], [" -Werror=unguarded-availability"])],
+            [],
+            [-Werror])
+
         LIPO_INTEL64_FLAGS=""
         if test "${enable_universalsdk}"
         then