rules.mk is stored in $(top_blddir). Without this change, VPATH builds stop working.

Add VPATH support to Apache: os/$(OS_DIR) is now equivalent to the old $(OS_DIR) Necessary include paths are added only in VPATH mode. Dependencies are stored in the build directory now. They contain paths which depend on the current build, and therefore they are not general. Fixed two dependencies in main/Makefile.in.

We want to build in the build dir, not the src dir. s/top_srcdir/top_builddir/

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)

Fix minor configuration & platform dependencies

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