m4: Create m4/ and update .gitignore files

We want to put third-party m4 scripts in this directory, but ignore the
autogenerated libtool scripts that show up after running autoreconf.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
m4: Split acinclude.m4 back into separate files under m4/

Autotools Mythbuster recommends using discrete files under m4/ instead
of combining everything into acinclude.m4.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Disable symbol versioning where unavailable

Non-GNU toolchains do not consistently support the --version-script ld
option.  Autodetect this at configure time and disable where appropriate.

fixes #9, closes #10

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>