autoconf: Helper options for adding GCC 32/64 bit flags

Two new configure options to add -m32 or -m64 to the CFLAGS and CXXFLAGS
when GCC is in use. By default, the user supplied options are
environment variables are respected, but these options are quick helps
for the common case of x86/x86_64 using GCC.

docs/autoconf.html

@@ -124,6 +124,13 @@ available for a few architectures. These will be used by default if
 one of these architectures is detected. This option ensures that
 assembly will not be used.
 </li>
+<li><code>--enable-32-bit, --enable-64-bit</code> - By default, the
+build will compile code as directed by the environment variables
+<code>CC</code>, <code>CFLAGS</code>, etc. If the compiler is
+<code>gcc</code>, these options offer a helper to add the compiler flags
+to force 32- or 64-bit code generation as used on the x86 and x86_64
+architectures.
+</li>
 </ul>
 </p>