autotools: use autoreconf & m4 dir to make it easier to add/manage macros

add m4/wxwin.m4 macro file from wxWidgets 2.9.5 (previously pasted into acinclude.m4)
remove obsolete WX* macros from acinclude.m4
move acinclude to m4/fso.m4

git-svn-id: svn://svn.icculus.org/fs2open@10224 387891d4-d844-0410-90c0-e4c51a9137d3
add wxfred2 execs to .gitignore

git-svn-id: svn://svn.icculus.org/fs2open@10225 387891d4-d844-0410-90c0-e4c51a9137d3