links: fix static link with directfb

links: fix static link with directfb

The AC_HAVE_FUNCS autoconf macro uses gcc to check for function existence.
This breaks when the library depends on libstdc++, like directfb. Add this
dependency externally.

Fixes:
http://autobuild.buildroot.net/results/548/548019b71c7db649f20d2c479bb8fb3612162d4a/
http://autobuild.buildroot.net/results/c94/c9495f54db388bde9d64325bb57fc4e58741f7a7/
http://autobuild.buildroot.net/results/9b8/9b883d42bff3633f16447d76d2eb4ddf68013788/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>