package/pulseaudio: allow to build without C++11 support

package/pulseaudio: allow to build without C++11 support

C++11 support is only required for WebRTC optional support but
pulseaudio can still be built with toolchains based on gcc 4.7 or older
witch doesn't have it.

Add a patch to make C++11 support optional and explicitely check for
gnu++11. Also stop the build only if no C++11 support has been found
and WebRTC support has been resquested.

Fixes:
http://autobuild.buildroot.net/results/d89/d897a94a8b6a52eba03d04c24536f554a7643000

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>