Add an --enable-reduced-exports configure option to link libmain.a

Add an --enable-reduced-exports configure option to link libmain.a
using ld's --whole-archive mode and avoid building exports.c entirely.
This reduces the size of a minimal httpd binary by 18% on Linux/x86_64
(687K to 560K) with no difference to the set of symbols available to
modules.

This option is only appropriate to use if using a shared libapr*
build, hence is non-default.

* configure.in: Add --enable-reduced-exports; disable httpdunit build
  if used.  Define AP_FORCE_EXPORTS if not enabled (default) in place
  of AP_USING_AUTOCONF.

* server/main.c (ap_suck_in_APR): Only build if AP_FORCE_EXPORTS is
  defined.

* Makefile.in: Link libmain.la using LIBMAIN_LIB.

* server/Makefile.in: Conditionally build exports.c into libmain.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1861685 13f79535-47bb-0310-9956-ffa450edef68