Make OCE version visible to OCC_VERSION_STRING_EXT

OCE version will be displayed for instance by DRAWEXE's dversion
command.
Add links to OCC website and release notes
Remove OCE_BUILD_TYPE

OCE_BUILD_TYPE does nothing, so remove it.
Fix build failures with Mesa 10.3.1

Newer Mesa looks buggy, several errors like this one are reported:
 /usr/include/GL/glxext.h:480:143: error: ‘GLintptr’ has not been declared
 typedef void ( *PFNGLXCOPYBUFFERSUBDATANVPROC) (Display *dpy, GLXContext readCtx, GLXContext writeCtx, GLenum readTarget, GLenum writeTarget, GLintptr readOffset, GLintptr writeOffset, GLsizeiptr size);
Work around this issue by defining GLX_GLXEXT_LEGACY.

Fix issue #534.
[mingw-warning-fix][missing-alloca]

alloca needs malloc.h under MinGW
Merge pull request #531 from tpaviot/review/occt-branding

Make OCE version visible to OCC_VERSION_STRING_EXT
Merge pull request #533 from tpaviot/review/remove-OCE_BUILD_TYPE

Remove OCE_BUILD_TYPE
Merge pull request #535 from tpaviot/review/fix-GLintptr

Fix build failures with Mesa 10.3.1
Merge pull request #538 from tpaviot/review/MinGW-OpenGl-fixes

Fix TKOpenGl MinGW compilation
Upgrade to OCCT 6.8.0

Remove OCE_DISABLE_BSPLINE_MESHER option: BRepMesh_FastDiscretFace had been
deeply modified in OCCTT 6.8.0, and keeping this patch around was not trivial.
Since we are not sure anymore that this option is really needed, it is now
removed.  Closes: #277.

New option: OCE_WITH_VTK

Clean up oce-config.h, many checks and cpp symbols have been removed in
OCCT 6.8.0.