Bug 1635271 - Restore CFLAGS to avoid repeat -fPIC flag when running configure after clobber. r=froydnj

If we don't restore CFLAGS, then after a clobber, there's no configure
cache, and therefore the CFLAGS get tweaked.  But running configure
again, the cache steps in and the extra -fPIC isn't added.  Locally
this means fewer sccache hits (but in automation, all builds at this
time are clobber builds, so there's no impact).

Differential Revision: https://phabricator.services.mozilla.com/D73757

js/src/old-configure.in

@@ -950,6 +950,7 @@ dnl availability attribute strict, so that compilation fails when the target is
                                    clock_gettime(CLOCK_MONOTONIC, &ts); ],
                                  ac_cv_clock_monotonic=$libs
                                  LIBS="$_SAVE_LIBS"
+                                 CFLAGS="$_SAVE_CFLAGS"
                                  break,
                                  ac_cv_clock_monotonic=no)
                     LIBS="$_SAVE_LIBS"

old-configure.in

@@ -1206,6 +1206,7 @@ dnl availability attribute strict, so that compilation fails when the target is
                                    clock_gettime(CLOCK_MONOTONIC, &ts); ],
                                  ac_cv_clock_monotonic=$libs
                                  LIBS="$_SAVE_LIBS"
+                                 CFLAGS="$_SAVE_CFLAGS"
                                  break,
                                  ac_cv_clock_monotonic=no)
                     LIBS="$_SAVE_LIBS"