Allow the LLD CMake build to work with -std=gnu++0x and -std=gnu++11 as
well as c++0x and c++11. This can be especially useful when building
with GCC in C++11 mode.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172897 91177308-0d34-0410-b5e6-96231b3b80d8