-complete the doc of the GSHHS Plugin
-compile GSHHS plugin even if matheval is not present

Added "File->Save Options" to automatically save options for the
current project (same name + appended ".opt")

OpenProject will detect the ".opt" and will load the options
automatically if available.

Adapted from an original idea and patch from <jiri.hnidek@tul.cz>.

Thanks Jiri!

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

that the 2 steps are orthogonal
* added full-hexa subdivision algo