Bugzilla Bug 180835

At least the 4.3.0 ebuild (didn't check anything older) uses those three
useflags to switch between internal and external copies of libraries. Unless
there is a specific upstream request to use internal libraries, the external
copy should always be used (and for what I can see in ./configure --help
output, the system copy is also the default).

Also, the mng useflag doesn't seem to actually disable mng support as far as I
can see.

HTH,
Diego

if you want to make the proper fixes, please do so. :)  I suppose this means
dropping png and jpeg and possibly zlib use flags (which isn't a problem in my
opinion).

Well, we could disable the three features with the useflags disabled, although
I'm not sure if it makes really sense.

I've decided to actually leaving the flags and fix them properly, after all one
*might* want to reduce its size for minimal X11 use, albeit not a common use of
it.

I also cleaned up qt_use to behave a bit more like use_enable/use_with,
accepting three arguments, this should make possible to rewrite the 'use'
checks in the ebuild through that function alone.