cairo package renaming, lablgl support for lablgtk2

Hello,
two commits :
* cairo : package rename. it is important, because opam use the directory name to install ocamlfind META package. Without change, cairo1.ocaml opam package installs "cairo" ocamlfind package in cairo sub directory, which could be fine, but "cairo.0.4.x" opam package get installed in same cairo sub directory...
to solve this, the best is to name opam package the same as ocamlfind package, that is to say : 
cairo1.ocaml-1.2.0 -> cairo.1.2.0 with cairo ocamlfind package (ocamlfind package not changed, only opam name);
cairo.0.4.4 -> cairo2.0.4.4 with cairo2 ocamlfind package (ocamlfind package not changed, only opam name)

* lablgtk2 : 
due to ocamlfind default install directory being in lib and not in lib/ocaml, (and lablGL -> lablgl renaming), it generates a problem with lablgl support in lablgtk2. solved with this commit