build: Leave M4 files to be installed by their parent packages.

And stop recommending bundling them in the README.txt.  It is expected
that someone regenerating the build system has all the required
packages such as pkg-config, bakefile, libsdl available.  As these
package provides their M4 macros themselves, they need not be
versioned here, which eases maintenance.

build/README.txt

@@ -4,23 +4,12 @@ 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 the updated configure and