The configure script incorrectly mangles `CFLAGS` by attempting to remove
`-Werror` from it, thus turning `-Werror=format-security` into just
`=format-security` which will cause subsequent autoconf tests to fail.

Patch out the custom `CFLAGS` mangling to fix the build.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>