configure: add CXX11_CXXFLAGS to LLVM_CXXFLAGS

configure: add CXX11_CXXFLAGS to LLVM_CXXFLAGS

Seemingly with LLVM7 and GCC 5.0, the former won't properly advertise
-std=c++11 and the latter will choke.

dd this temporary workaround, otherwise we'll get errors like:

In file included from /usr/include/c++/5/type_traits:35:0,
                 from /usr/lib/llvm-7/include/llvm/Support/type_traits.h:18,
                 from /usr/lib/llvm-7/include/llvm/ADT/Optional.h:22,
                 from /usr/lib/llvm-7/include/llvm/ADT/STLExtras.h:20,
                 from /usr/lib/llvm-7/include/llvm/ADT/StringRef.h:13,
                 from /usr/lib/llvm-7/include/llvm/Target/TargetMachine.h:17,
                 from ../../../src/amd/common/ac_llvm_helper.cpp:36:
/usr/include/c++/5/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>