cmake: Set the only working C++ standard on Cygwin

Gcc on Cygwin poorly supports std=c++11 and c++0x, thus we use gnu++11
on this platform. Gnu++11 does not work in Clang, so we don't want to
remove the previous flag entirely.

Change-Id: Iaffd0844680d7c9321dd23ccf4ae9a84b83665fe
cmake: Add a possibility to disable building the documentation

Change-Id: I8e0aa61c78b2e854f380bb349426f4f92dac6665