package/directfb: disable for static builds

DirectFB is written in C++, but can be linked to by a C library or
program.

When doing shared link, this is fine because the linker gets help from
the DT_NEEDED flags and knows what libraries to pull in during the link.

However, during a static link, the linker does not get such help.

Properly fixing this would require that there is support in pkg-config
and autotools to specify that the C++ runtime must be linked. Alas there
is no sush support. The only option is to add -lstdc++ to the
Libs.Private field in directfb.pc. But this is not upstreamable, because
there are other C++ runtimes in the wild (e.g. -lc++ from llvm/clang).

However, DirectFB in a static scenario is probably not a very common
scenario.

Disable DirectFB for static builds.

Fixes:
    http://autobuild.buildroot.org/results/3d3/3d3036d40ddad71d872d910aae7a24975706d2e9/
    http://autobuild.buildroot.org/results/d1c/d1c35a6003396942b584f2f2a5e8bf4ac2fbe370/
    http://autobuild.buildroot.org/results/d45/d4504871bd47930e8363032d380cdfcc5bb8aee7/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>