Improve pkg-config usage for mod_ssl/ab:

also use pkg-config for determining the -l flags (and fall back
to a hardcoded default of "-lssl -lcrypto")

add --static to pkg-config invocations, so that libraries for
static linking are also taken into account (PR 54252 - note that
the additional flags will only appear in modules/ssl/modules.mk
and ab_LDFLAGS, so potential side effects are limited)

separate --libs-only-L and --libs-only-other into two invocations
(can't be used concurrently, only the first takes effect)

use --silence-errors where applicable


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1428184 13f79535-47bb-0310-9956-ffa450edef68