Wextra results in a dreadful wall of yellow in the test-suite and is
scary when using CGAL. Reduce some of the noise by silencing the
obvious cases with CGAL_USE or comments.
Enforced CGAL_NDEBUG rule:
- code under include/CGAL/ and src/ must not use NDEBUG and assert(), but CGAL_NDEBUG and CGAL assertion macros.
- code under demo/, examples/ and test/ must use NDEBUG and assert().
* CGAL_error to CGAL_error_msg
* introduced a macro CGAL_error()
* added some words about CGAL_error to the developers manual
* renamed most of assert(x) into CGAL_assertion(x)
* renamed exit(x) with x != 0 , CGAL_assertion(false) and assert(false) into CGAL_error
* CORE left untouched, OpenNL changed