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.