Bug#500559: Should be able to specify custom lablgtk2 location

[Martijn Vermaat <mvermaat@cs.vu.nl>]

Package: liblablgtksourceview-ocaml
Version: 0.2.1-2

Building the lablgtksourceview source should be possible with a non-standard
location of the lablgtk2 files.

For example, the libcairo-ocaml package looks for the LABLGTKDIR environment
variable.

The liblablgtksourceview-ocaml package just looks in the standard locations
and fails:

  mvt600@fspc459:~/lablgtksourceview-0.2.1$ LABLGTKDIR=/home/mvt600/public/usr/lib/ocaml/3.09.2/lablgtk2 ./configure --prefix=/home/mvt600/public
  [...]

  ### checking required tools and libraries ###
  checking for Lablgtk2 (byte) default install... no
  Could not link with LablGtk2
  You way have a look at config_check.log for details.

Suggested fix: configure should honour the LABLGTKDIR environment variable,
if set.