Merge r1812263, r1812301 from trunk:

Merge r1812263, r1812301 from trunk:

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.

Follow up to r1812263.

As suggested by Joe, add --maintainer/debugger-mode's CFLAGS in
NOTEST_CFLAGS to avoid interractions with autoconf's AC_LANG_PROGRAM.

APACHE_ADD_GCC_CFLAG now also forces -Wno-strict-prototypes for -Werror
to work despite AC_LANG_PROGRAM generating this warning by itself.

Submitted by: rjung, ylavic
Reviewed by: ylavic, rjung, jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1812437 13f79535-47bb-0310-9956-ffa450edef68