bpo-38472: setup.py uses LC_ALL=C to check the C compiler (GH-30929)

Fix GCC detection in setup.py when cross-compiling. The C compiler is
now run with LC_ALL=C. Previously, the detection failed with a German
locale.
(cherry picked from commit a9503ac)

Co-authored-by: Victor Stinner <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Build/2022-01-26-22-59-12.bpo-38472.RxfLho.rst

@@ -0,0 +1,2 @@
+Fix GCC detection in setup.py when cross-compiling. The C compiler is now
+run with LC_ALL=C. Previously, the detection failed with a German locale.

setup.py

@@ -757,7 +757,9 @@ def add_cross_compiling_paths(self):
         tmpfile = os.path.join(self.build_temp, 'ccpaths')
         if not os.path.exists(self.build_temp):
             os.makedirs(self.build_temp)
-        ret = run_command('%s -E -v - </dev/null 2>%s 1>/dev/null' % (CC, tmpfile))
+        # bpo-38472: With a German locale, GCC returns "gcc-Version 9.1.0
+        # (GCC)", whereas it returns "gcc version 9.1.0" with the C locale.
+        ret = run_command('LC_ALL=C %s -E -v - </dev/null 2>%s 1>/dev/null' % (CC, tmpfile))
         is_gcc = False
         is_clang = False
         in_incdirs = False