COMP: reduce compiler warnings for gcc-7

- ignore implicit-fallthrough for ragel generated code.

- add -Wno-deprecated-declarations for c++LESSWARN.
  These principally associated with older CGAL versions and their use
  of particular mpfr routines.

src/fileFormats/stl/STLAsciiParseRagel.rl

@@ -33,6 +33,9 @@ Description
 #include "STLReader.H"
 #include "OSspecific.H"
 
+// Ragel switch/case logic may have several implicit fallthroughs
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
 // https://en.wikipedia.org/wiki/STL_%28file_format%29#ASCII_STL
 //
 // Format