Fix all "uninitialized var" gcc warnings, even if they are bogus.

Fix some dependencies + enable GSL (so that more people test it).

This is a pretty big commit.

- The Numerical Recipes routines have been moved to their own
subdirectory: NR/

- These routines are now completely optional. Configure will
determine if they have to be compiled in (if it's a non-free version
of Gmsh) or if we should use the GSL routines instead.

- The GSL routines are there. Well, sort of. The SVD stuff works
great, but the minimization routines still need more work. The whole
infrastructure is there, though: we can already compile a fully GPL
compliant version of Gmsh by removing the NR subdirectory (or by
specifying --enable-gsl to configure).

- I also simplified the options passed to the compiler (OPTIM and
FLAGS).

That's all for today folks...

Clean up VERSION_FLAGS and OS_FLAGS (use HAVE_FLTK instead of _FLTK,
remove _BLACKBOX, etc.)

added ReadImg.* into Graphics
modified Makefiles (now libfltk_images.a is used)
This is a beta version of image loading with gmsh

New makefile system based on autoconf. Only tested on Linux right now.

Rename Mesh/2D_Mesh_Shewchuk.cpp -> Mesh/2D_Mesh_Triangle.cpp

Make Triangle optional (license issues)

Added missing return

Updated copyright

Create geometry points from physical points in Read_Mesh.

Introduce OpenGL display lists for drawing meshes. Does not seem to
work much faster on my machine, though.

drawing. We are still very slow, though:

- we should preprocess the mesh to draw large triangle/line strips
- we should store the normals (and NOT IN A SET!!)
- we should store the elements in a structure allowing a faster traversal
- we should do all this in single precision

The same remarks apply for the post-processing stuff. Worse, in
post-pro, due to Get_Coord and SaturateValues, we basically copy ALL
THE DATA SET at least one per drawing, even when we don't have to (no
Raise/Offset/Saturate)... This is horrible, and makes us lose all the
advantage of the fast list_PointerFast stuff I designed in the forst
place. Ahhh, the cost of generality...

L'introduction d'une relaxation dans XYtoUV permet de resoudre pas mal de problemes (cf. bench/bugs/xy2uv