Bug 1604881 - fail if user attempts to set CC or CXX with "mk_add_options" in .mozconfig r=firefox-build-system-reviewers,chmanchester

Differential Revision: https://phabricator.services.mozilla.com/D57656

--HG--
extra : moz-landing-system : lando

build/moz.configure/old.configure

@@ -22,6 +22,15 @@ def autoconf(mozconfig, autoconf):
                              assignment)
                 if m:
                     mozconfig_autoconf = m.group(1)
+                # Check whether we've exported any known-broken variables.
+                m = re.match('(?:export\s+)?(?:CC|CXX)\s*:?=\s*(?:.+)$',
+                             assignment)
+                if m:
+                    die('Setting the value of CC or CXX with "mk_add_options" '
+                        'can cause the build to fail unexpectedly. Please '
+                        'change your mozconfig to instead use '
+                        '"ac_add_options", e.g. ac_add_options '
+                        'CC=my-custom-cc".')
 
     autoconf = autoconf[0] if autoconf else None