lang/gcc(46,47,48,49,5): Use OPTIONS_EXCLUDE_DragonFly to block JAVA

The JAVA frontend doesn't build on DragonFly on any release. The new
OPTIONS_EXCLUDE_${OPSYS} feature is a nice way to avoid the use of
Makefile.DragonFly (most are in dports, but one is in lang/gcc5).

The recent addition of CXXFLAGS to lang/gcc5 prevents Makefile.DragonFly
on lang/gcc5 from being removed outright. There are a couple of options
available to allow its removal, but I'll need to discuss with Gerald.

Approved by: DragonFly blanket