Removed GSL dependency. Gmsh now uses blas and lapack for linear algebra
stuff.

moved all string options to std::string

*****************************************************************
WARNING: this has the potential to break things in subtle ways...
         PLEASE TEST AND REPORT ANY WEIRDNESSES
*****************************************************************

get string option would NEVER work! Ugly fix until will replace all const char*
with std::strings in the options

HAVE_XXX defines.

This fixes three longstanding problems:

- the API was hard to use because some headers had HAVE_XXX ifdefs, so
  that cient codes had to know how the lib was compiled to compile
  themselves

- we had to do make clean after each ./configure, which erased too
  much (e.g. most of contrib)

- make depend could include unwanted stuff since it redefined FLAGS

- fix bb computation to avoid model jumping around when adding points in z=0 plane
- pass interpol matrices as ref

- added automatic transfinite corner selection for volumes

window with experimental tree browser

multi-window & multi-coordinate system drawing.

There's no new functionality for now: PLEASE TEST THAT EVERYTHING
WORKS AS IT USED TO BEFORE.

allow recombine, transfinite and embedding constraints on GModel entities
in parser

- fixed missing getShapeFunction/getGradShapeFunction + return all sf
  at once so we can implement MElement in term of these consistently
  (w or w/o FunctionSpace)

- can now merge multiple meshes and remove duplicate mesh vertices
  (useful e.g. when merging several STLs)

- added easy interface to create model-based postpro datasets on the
  fly + use this for mesh quality stats