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([enabled])

Use better default, but allow configure flag to remain.