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

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

* no lc at points == MAX_LC (-> lc/10 like for STEP & co)
* don't specify a char length at points by default when creating a geo with the GUI

corrected bug for higher order mappings in post views (supports q up to 4)

****************************************************************
**** PLEASE TEST YOUR SCRIPTS (ESPECIALLY ONES WITH LISTS, *****
****        COMPLICATED AFFECTATIONS, ETC.)                *****
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orientation in extrusions