Enable GObject Introspection support

Most GObject things work, but some non-GObject things need API change to work.
Remove unused CFLAGS and LIBS. Allow using libarchive without a pc file.

Unused CFLAGS and LIBS may cause strange error in some system. FreeBSD system
libarchive does not have a pc file installed. Use AC_SEARCH_LIBS to find it,
so FreeBSD users do not have to install libarchive from ports.