[cmake] Use -std=gnu++11 to fix alloca() on NetBSD

Prefer using '-std=gnu++11' over '-std=c++11' when available, as NetBSD
exposes the correct alloca() implementation only with gnu* C/C++
standards.

Differential Revision: https://reviews.llvm.org/D55477

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348854 91177308-0d34-0410-b5e6-96231b3b80d8