Revert "Remove unnecessary use of arguments from AC_ARG_WITH([extra-cflags])."

This reverts commit 6a917dcd1378e2a50ece9f007e79a8a5058a59cb.
Revert "Allow adding extra CFLAGS with a configure option."

This reverts commit 64b3cefbdbc0d6deef1dd3bcde890e273c323168.

Apparently, setting CFLAGS won't override the preset AM_CFLAGS
afterall. Not sure why this was giving me a problem before.