omega-h: fix -std=c++11 build error (#8543)

cd /tmp/balay/spack-stage/spack-stage-joxtpr/omega_h-9.13.4/spack-build/src && /home/balay/spack/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/openmpi-3.1.0-7ws4tgx4z3o
ru5g5skjrxs5sobpeiyef/bin/mpic++   -Domega_h_EXPORTS -I/home/balay/spack/var/spack/stage/omega-h-9.13.4-th4b67fz5es6cbgcar742jxamjt3tqjb/omega_h-9.13.4/src -I/tmp/bala
y/spack-stage/spack-stage-joxtpr/omega_h-9.13.4/spack-build/src -I/home/balay/spack/var/spack/stage/omega-h-9.13.4-th4b67fz5es6cbgcar742jxamjt3tqjb/omega_h-9.13.4/tpl
-I/home/balay/spack/opt/spack/linux-ubuntu16.04-x86_64/gcc-5.4.0/zlib-1.2.11-5nus6knzumx4ik2yl44jxtgtsl7d54xb/include  -O2 -g -DNDEBUG -fPIC   -o CMakeFiles/omega_h.di
r/Omega_h_map.cpp.o -c /home/balay/spack/var/spack/stage/omega-h-9.13.4-th4b67fz5es6cbgcar742jxamjt3tqjb/omega_h-9.13.4/src/Omega_h_map.cpp
In file included from /usr/include/c++/5/chrono:35:0,
                 from /home/balay/spack/var/spack/stage/omega-h-9.13.4-th4b67fz5es6cbgcar742jxamjt3tqjb/omega_h-9.13.4/src/Omega_h_timer.hpp:4,
                 from /home/balay/spack/var/spack/stage/omega-h-9.13.4-th4b67fz5es6cbgcar742jxamjt3tqjb/omega_h-9.13.4/src/Omega_h_timer.cpp:1:
/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.
 #error This file requires compiler and library support \
  ^

var/spack/repos/builtin/packages/omega-h/package.py

@@ -65,3 +65,9 @@ def cmake_args(self):
         else:
             args.append('-DTPL_ENABLE_ZLIB:BOOL=OFF')
         return args
+
+    def flag_handler(self, name, flags):
+        flags = list(flags)
+        if name == 'cxxflags':
+            flags.append(self.compiler.cxx11_flag)
+        return (None, None, flags)