directfb: mark as available only for gcc >= 4.5

The new DirectFB version does not build with gcc 4.3 from the Blackfin
toolchain. One of the reason is that va_copy has some issues, which
were fixed in gcc 4.4.0
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36799). There are also
some other issues, which were fixed by a patch proposed by Peter
Seiderer at
http://lists.busybox.net/pipermail/buildroot/2015-February/120281.html

.

However, it probably doesn't make a lot of sense to carry patches that
are not upstream for such old compilers. Instead, this commit takes
the action of making DirectFB available only on toolchains using gcc
>= 4.5, which was tested with the Arago toolchain. gcc 4.4 could
potentially work, but wasn't tested (it is no longer supported by the
internal toolchain backend, and we don't have any toolchain based on
gcc 4.4), so we take the safe decision of requiring at least gcc 4.5.

[Peter: add comment explaining toolchain dependenc as suggested by Vincente]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>