Minor fix

1. type_vec4.hpp
__declspec(align(16)) produces a compiler error on VS2012:
func_common.inl(634): error C2719: 'a': formal parameter with
__declspec(align('16')) won't be aligned
core_func_common.cpp(310) : see reference to function template
instantiation 'glm::detail::tvec4<T,P> glm::mix<float,highp>(const
glm::detail::tvec4<T,P> &,const glm::detail::tvec4<T,P>
&,glm::detail::tvec4<bool,highp>)' being compiled
with
[
T=float,
P=highp
]
and a warning on CygWin using gcc 4.7.2:
type_vec4.hpp:40:31: warning: 'align' attribute directive ignored
[-Wattributes]

2. gtx_bit.cpp
glm::uint32 x_max = 1 << 13;
glm::uint32 y_max = 1 << 12;
result out of memory on my machine.
Merge pull request #91 from zhumeng1989/0.9.5

Minor fix
Better compiler detection in CMake. Language detection for Clang
Added error for unsupported compiler versions
Fixed merge
Merge branch '0.9.5' into packing
Merge branch 'packing' of https://github.com/g-truc/glm into packing