Build: simplify and update deprecated libtool/ltdl integration

To name a few pain points in the former version:

• pacemaker is not distributed with "libltdl.tar" for the past few years and it's unclear if it ever was or if it's just the heartbeat legacy; definitely no recipe to prepare such a file was found
• custom mechanism to use bundled libltdl is redundant (and inferior in parameterization) compared to options automatically available to configure script thanks to newly introduced LTDL_INIT macro (--with-ltdl-lib DIR, --with-included-ltdl)
• "INCLTDL" was likely a typo for "LTDLINCL"