Fix maintainer mode with GCC/Clang.

Fix maintainer mode with GCC/Clang.

Setting -Wstrict-prototypes in combination
with -Werror leads to compiler errors during
configure checks (autoconf generates incomplete
prototypes).

Adding -Wno-error=strict-prototypes lets the
compiler tolerate those.

Possible future enhancement: remember such
"configure time only" flags and remove them
from CFLAGS before generating our build time
files (Makefile, config_vars.mk etc.), so that
the full -Werror is in place during building.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1812263 13f79535-47bb-0310-9956-ffa450edef68