A backslash should not be allowed inside a quoted string unless
it is used to escape another character.
This fixes https://savannah.gnu.org/bugs/?61389.
Bug existed since each of these syntaxes was introduced.
(Awk and Fortran do not know include files with names between
angled brackets, so those regexes are dropped in the bargain.)
Fortran and POV-Ray files are not found on any regular install of a
Unixy system nor in common software packages. People who want these
syntaxes will have to make a little effort to enable them.