Added simple instructions and autogen.mk to rebuild configure script.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35626 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

build/README.txt

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+autogen.mk
+
+autogen.mk is a makefile provided to automatically update the GNU autotools
+build system.  It will run Bakefile, aclocal, and autoconf as appropriate.
+
+The .m4 files required for running aclocal are located in build/aclocal.
+When upgrading software that wxWidgets depends on (for example, Bakefile,
+cppunit, GTK, SDL, or others) it is advisable to upgrade the .m4 files
+located in the build/aclocal directory.
+
+It is particularly important that if you use a newer version of Bakefile
+to generate the Makefile.in files that you use the newer bakefile*.m4.
+Because build/autogen.mk is a Makefile it will automatically rerun
+aclocal and autoconf as necessary whenever any m4 in build/aclocal is
+newer than the generated aclocal.m4.
+
+You can achieve this simply by copying the new bakefile*.m4 files from 
+PREFIX/share/aclocal/ into the build/aclocal/ directory and
+rerunning make -f build/autogen.mk.  Note that you should _not_ preserve
+source file times (don't use cp -p) or else it's possible your .m4 files
+will be older than the generated aclocal.m4).
+Example:
+cp /usr/share/aclocal/bakefile*.m4 build/aclocal/
+make -f build/autogen.mk
+
+Please don't forget to commit updated .m4 files as well as updated aclocal.m4,
+configure, and Makefile.in files to wxWidgets.

build/autogen.mk

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+# Created: 2005/03/12
+# Author: David Elliott
+
+# Usage Example:
+#  make -f build/autogen.mk ACLOCAL=aclocal-1.9
+
+# This is a simple Makefile to update the UNIX build system in such a
+# way that doing a cvs diff on its output files should reveal only the
+# true changes, not meaningless differences due to slightly different
+# autoconf or aclocal m4 files.
+
+# For aclocal: All necessary m4 files are located in the build/aclocal
+# directory.  Running aclocal -I build/aclocal when using aclocal 1.9
+# will result in an aclocal.m4 which uses m4_include for these files.
+
+ACLOCAL=aclocal
+AUTOCONF=autoconf
+BAKEFILE_GEN=bakefile_gen
+AUTOHACKS_PREPEND_INCLUDE_DIR=build/autoconf_prepend-include
+
+# configure depends on everything else so this will build everything.
+.PHONY: all
+all: configure
+
+.PHONY: autoconf_m4f
+
+# Invoke make on wxAutohacks dir, but don't fail if it's not present
+autoconf_m4f:
+	-make -C $(AUTOHACKS_PREPEND_INCLUDE_DIR)
+
+BAKEFILES=\
+  build/bakefiles/wx.bkl \
+  build/bakefiles/common.bkl \
+  build/bakefiles/files.bkl \
+  build/bakefiles/monolithic.bkl \
+  build/bakefiles/multilib.bkl \
+  build/bakefiles/opengl.bkl \
+  build/bakefiles/plugins.bkl \
+  build/bakefiles/build_cfg.bkl
+
+# Run bakefile-gen (which generates everything) whenever a bakefile is newer
+# than Makefile.in or autoconf_inc.m4.
+# This dep is obviously wrong but probably close enough
+autoconf_inc.m4 Makefile.in: $(BAKEFILES)
+	cd build/bakefiles && \
+	$(BAKEFILE_GEN) -f autoconf
+
+# Run configure whenever configure.in, aclocal.m4 or autoconf_inc.m4 is updated
+# Depend on our custom autoconf.m4f
+configure: configure.in aclocal.m4 autoconf_inc.m4 autoconf_m4f
+	$(AUTOCONF) -B $(AUTOHACKS_PREPEND_INCLUDE_DIR)
+
+ACLOCAL_SOURCES = \
+  build/aclocal/bakefile.m4 \
+  build/aclocal/bakefile-lang.m4 \
+  build/aclocal/cppunit.m4 \
+  build/aclocal/gst-element-check.m4 \
+  build/aclocal/gtk-2.0.m4 \
+  build/aclocal/gtk.m4 \
+  build/aclocal/pkg.m4 \
+  build/aclocal/sdl.m4
+
+# Run aclocal whenever acinclude or one of our local m4s is updated.
+aclocal.m4: configure.in acinclude.m4 $(ACLOCAL_SOURCES)
+	$(ACLOCAL) -I build/aclocal
+