Use pkg-config to get libsmbclient CFLAGS

Hi,

This fix install of pysmbc on my debian testing where smbclient headers files are in /usr/include/samba-4.0

pkg-config (when availiable) can help fixing build on several OS setup/versions.

I also try to use Extension.extra_compile_args but distutils put my custom cflags at the end of the gcc command and it doesn't work...