Replace USE_AUTOTOOLS=libltdl with an ordinary LIB_DEPENDS in all ports.
There are only 60 such ports so there doesn't need to be a separate
keyword or USES for this.

Approved by:	portmgr (bapt)
- Remove last uses of USE_AUTOTOOLS=libtool from bsd.gnome.mk
  (lthack, ltasneededhack and ltverhack) [1]
- Remove support for USE_AUTOTOOLS=libtool and USE_AUTOTOOLS=libltdl

PR:		188978 [1]
Approved by:	portmgr (bapt)