linux // added clang++ support

[snikulov@fedora20 b]$ CC=clang CXX=clang++ cmake .. -DBUILD_UT=ON -DBUILD_SPDY=ON
....
[snikulov@fedora20 b]$ make
...
In file included from /home/snikulov/work/github/pion/include/pion/hash_map.hpp:21:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/unordered_map:35:
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/bits/c++0x_warning.h:32:2: error: This file requires compiler and library
      support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11
      compiler options.
#error This file requires compiler and library support for the \

To fix this updated PionUtils.cmake to handle clang compiler id and set option "-std=c++11"