[gl2ps] Compiler warnings

[Jordi Gutiérrez Hermoso]

Please cc me when responding, as I'm not subscribed to the gl2ps mailing list.

I'm seeing some warnings when compiling gl2ps.c, because it's using
the "index" identifier for some variables. This conflicts with a
legacy index(3) function in some POSIX implementations defined in
strings.h. It's a mostly harmless warning, but it would be nice to fix
it. It could also conceivably be a compiler bug, as this function
shouldn't be defined from string.h (notice the singular, not plural
"string"), which is the only header that gl2ps.c #includes.

Should something be done about this?