iNo need to set CFLAGS and LDFLAGS for "ab"

during detection of libnghttp2. "ab" doesn't
yet support http2.

The macros were probably copied over from our
OpenSSL detection, where it actually makes
sense to adjust the "ab" flags.

Backport of r1742460 from trunk.

CTR


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1742463 13f79535-47bb-0310-9956-ffa450edef68