Revert "Add FreeBSD support."

This reverts commit 0891005f748792c63d32f798b656393f43f16d00.

Those things should be handled by FreeBSD ports system as they are currently.
Description how to handle those thing manually are written at LXDE Wiki site, see
http://wiki.lxde.org/en/LXDE:PCManFM_build_and_setup_guide#Compile_and_Install_-_FreeBSD
Rename configure.in -> configure.ac : it's deprecated since autoconf 2.50.