Spin off module-specific build options into separate build vars.

Spin off module-specific build options into separate build vars.
[second try, supersedes r1385214]

Add MOD_CFLAGS, MOD_LDFLAGS variables etc. to the build commands,
which are available to modules for customization on a per-subdir
basis (by adding definitions to modules.mk). Reduces the risk
of side-effects when a module needs to add CFLAGS, LDFLAGS etc.
and these would be added to the global settings (ALL_CFLAGS etc.).

Adapt build settings for mod_ssl, mod_socache_dc, mod_deflate,
mod_xml2enc, mod_proxy_html, and mod_lua to use the new MOD_xxx
build variables.

Change PICFLAGS, SHLTCFLAGS and LTCFLAGS into config vars, instead
of AC_SUBSTing them in build/rules.mk.in. For support/ab, introduce
ab_CFLAGS and ab_LDFLAGS, and define explicit make targets where
they appear at the proper position in the build commands.

Consistently use "--with-xxx=PATH" in configure help strings which
are used to specify a path to the installation directory of an
auxiliary package.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1385216 13f79535-47bb-0310-9956-ffa450edef68