ltdl, use LTDL_INIT's "recursive" mode

This decreases the distdir size about 1.3 MB and increases configure
speed.
Note we also removed unused check for ltdl.h
bldchn, AC_CONFIG_HEADER is obsolete since a long time
ltdl, use LTDLINCL instead of obsolete INCLTDL
ltdl, add make deps on included libltdl
Merge remote-tracking branch 'rudi/bld/cosmetics'

* rudi/bld/cosmetics:
  ltdl, add make deps on included libltdl
  ltdl, use LTDLINCL instead of obsolete INCLTDL
  bldchn, AC_CONFIG_HEADER is obsolete since a long time
  ltdl, use LTDL_INIT's "recursive" mode