add info about build system macros

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+PHP Build System V5 Overview
+
+- supports Makefile.ins during transition phase
+- not-really-portable Makefile includes have been eliminated
+- supports seperate build directories without VPATH by using
+  explicit rules only
+- does not waste disk-space/CPU-time for building temporary libraries
+  => especially noticeable on slower systems
+- slow recursive make replaced with one global Makefile
+- eases integration of proper dependencies
+- adds PHP_DEFINE(what[, value]) which creates a single include-file
+  per what.  This will allow more fine-grained dependencies.
+- abandoning the "one library per directory" concept
+- improved integration of the CLI
+- several new targets
+  build-modules: builds and copies dynamic modules into modules/
+  install-cli: installs the CLI only, so that the install-sapi
+               target does only what its name says
+- finally abandoned automake (still requires aclocal at this time)
+- changed some configure-time constructs to run at buildconf-time
+- upgraded shtool to 1.5.4
+- removed $(moduledir) (use EXTENSION_DIR)
+
+The Reason For a New System
+
+It became more and more apparent that there is a severe need
+for addressing the portability concerns and improving the chance
+that your build is correct (how often have you been told to
+"make clean"? When this is done, you won't need to anymore).
+
+
+If You Build PHP on a Unix System
+
+
+You, as a user of PHP, will notice no changes.  Of course, the build
+system will be faster, look better and work smarter.
+
+
+
+If You Are Developing PHP
+
+
+
+
+Extension developers:
+
+Makefile.ins are abandoned.  The files which are to be compiled
+are specified in the config.m4 now using the following macro:
+
+PHP_NEW_EXTENSION(foo, foo.c bar.c baz.cpp, $ext_shared)
+
+E.g. this enables the extension foo which consists of three source-code
+modules, two in C and one in C++.  And dependending on the user's
+wishes, the extension will even be built as a dynamic module.
+
+The full syntax:
+
+PHP_NEW_EXTENSION(extname, sources [, shared [,sapi_class[, extra-cflags]]])
+
+Please have a look at acinclude.m4 for the gory details and meanings
+of the other parameters.
+
+And that's basically it for the extension side.
+
+If you previously built sub-libraries for this module, add
+the source-code files here as well.  If you need to specify
+separate include directories, do it this way:
+
+PHP_NEW_EXTENSION(foo, foo.c mylib/bar.c mylib/gregor.c,,,-I@ext_srcdir@/lib)
+
+E.g. this builds the three files which are located relative to the
+extension source directory and compiles all three files with the
+special include directive (@ext_srcdir@ is automatically replaced).
+
+Now, you need to tell the build system that you want to build files
+in a directory called $ext_builddir/lib:
+
+PHP_ADD_BUILD_DIR($ext_builddir/lib)
+
+Make sure to call this after PHP_NEW_EXTENSION, because $ext_builddir
+is only set by the latter.
+
+If you have a complex extension, you might to need add special
+Make rules.  You can do this by calling PHP_ADD_MAKEFILE_FRAGMENT
+in your config.m4 after PHP_NEW_EXTENSION.
+
+This will read a file in the source-dir of your extension called
+Makefile.frag.  In this file, $(builddir) and $(srcdir) will be
+replaced by the values which are correct for your extension
+and which are again determined by the PHP_NEW_EXTENSION macro.
+
+Make sure to prefix *all* relative paths correctly with either
+$(builddir) or $(subdir).  Because the build system does not
+change the working directory anymore, we must use either
+absolute paths or relative ones to the top build-directory.
+Correct prefixing ensures that.
+
+
+SAPI developers:
+
+Instead of using PHP_SAPI=foo/PHP_BUILD_XYZ, you will need to type
+
+PHP_SELECT_SAPI(name, type, sources.c)
+
+I.e. specify the source-code files as above and also pass the
+information regarding how PHP is supposed to be built (shared
+module, program, etc).
+
+For example for APXS:
+
+PHP_SELECT_SAPI(apache, shared, sapi_apache.c mod_php4.c php_apache.c)
+
+
+
+General info
+
+The foundation for the new system is the flexible handling of
+sources and their contexts.  With the help of macros you
+can define special flags for each source-file, where it is
+located, in which target context it can work, etc.
+
+Have a look at the well documented macros
+PHP_ADD_SOURCES(_X) in acinclude.m4.