geda_gschem and libgeda

Hi Savant.

ive played around a bit with the gschemnewstsh branch and i've found a
lot of geda symbol data in that branch (which is bad).

there's no longer need for this. i've got a fix in gschem+libgeda on
top of your branch (on tool):

- use PKG_CHECK_MODULES to find out if gEDA is installed
  if yes: use it to find out where the datadir is
  if no: use a dummy data dir path
- configure --with-geda=<otherpath> overrides that path (and skips
  looking ouy)
- the change to lang_gschem.cc should be self explainatory. note that
  gEDA uses $datadir/sym for symbols, not 'symbols'
- libgeda is now acessible from within lang_gschem.cc (is this useful?)

also i've added geda_gschem.cc to LANG_SRCS in src/Makefile.am, as
currently no shared-object compilation/installation is implemented in
that branch (see gnucap-uf for an approach to installing modules).

have fun
felix