Revert "configure: ignore external CFLAGS when debug is enabled"

This reverts commit 80ad13d53677a1c0383f58c360dacf05c838579c.
configure: cleaned up syntax formatting
configure: added option --enable-minimal-flags

This option disables all modifications to the CFLAGS that are not
required, including special debugging flags. This option is useful
for certain packaging systems (ex: Gentoo's portage).