use gcc visibility attributes and some other build updates (take2)

* use gcc visibility attributes
* define MODPLUG_NO_FILESAVE in modplug.h too
* require 10.5+ for osx/x86_64 builds
* add -fno-common among clfags
* use a m4 directory for *.m4
* add win32-dll option to LT_INIT
* disable maintainer mode by default
* disable AGC and FILESAVE for all targets for uniformity. define MODPLUG_NO_FILESAVE in modplug.h too, matching stdafx.h. minor tidy-ups.