[3.8] bpo-37437: Pass -Wno-unreachable-code when compiling expat. (GH-14473)

(cherry picked from commit 95da310)

Co-authored-by: Benjamin Peterson <[email protected]>

Author: benjaminp

setup.py

@@ -1599,9 +1599,9 @@ def detect_expat_elementtree(self):
 
             cc = sysconfig.get_config_var('CC').split()[0]
             ret = os.system(
-                      '"%s" -Werror -Wimplicit-fallthrough -E -xc /dev/null >/dev/null 2>&1' % cc)
+                      '"%s" -Werror -Wno-unreachable-code -E -xc /dev/null >/dev/null 2>&1' % cc)
             if ret >> 8 == 0:
-                extra_compile_args.append('-Wno-implicit-fallthrough')
+                extra_compile_args.append('-Wno-unreachable-code')
 
         self.add(Extension('pyexpat',
                            define_macros=define_macros,