re-write load_umx:
Replace the linear search, retrieve the offset, size and
object type directly from umx file by parsing the upkg.
Old code used to fail loading umx files from Unreal beta,
for example.
disable AGC and FILESAVE for all targets for uniformity. define MODPLUG_NO_FILESAVE in modplug.h too, matching stdafx.h. minor tidy-up.
require 10.5+ for osx/x86_64 builds
add -fno-common among cflags
add win32-dll option to LT_INIT
use gcc visibility attributes
use a m4 directory for *.m4
disable maintainer mode by default (add AM_MAINTAINER_MODE to configure.ac.)
Use AM_MAINTAINER_MODE([enable])

Use better default, but allow configure flag to remain.
Merge pull request #9 from sezero/szo3

re-write load_umx