buildroot

libcgi: add patch to not require C++ support

libcgi is written in C, there is no need for a C++ compiler, but CMake
by default assumes that both C and C++ are needed. To fix that, this
commit adds a patch, submitted upstream, to adjust the CMakeLists.txt.

Fixes:

  http://autobuild.buildroot.net/results/d0b4740bdbd84ed6fcbba706caf42ebba4c07d15/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>