Change the configure-based MPM build mechanism to support building an MPM as a shared shared or dynamic module, primarily using the APACHE_MPM_MODULE() function. --enable-mpms-shared now builds/installs the MPMs as dynamic modules. (But no LoadModule directives are added.)

Overall UNIX build system improvements: * Makefile header is now completely dynamic * Absolute paths everywhere (fixes Tru64 support) * Get rid of LTLIBRARY_SHARED_NAME rule in library.mk (fixes Irix support, untested) * VPATH does not contain variables anymore (fixes UnixWare support) * Remove inclusion of program.mk in support/Makefile.in (PROGRAM_NAME is empty => breaks Irix and others) * Call REENTRANCY_FLAGS earlier, so that flags are passed to header checks (might fix OpenBSD pthread.h-detection, untested)

Minor nit: use top_srcdir instead of topsrcdir, to be more consistent with autoconf.

These changes are committed together, because they depend on each other. - shared modules can be built in the tree - added support for --with-layout, uses APACI's config.layout - working 'make install' - working 'make depend' - working Pthreads checks - buildconf replaced