fixed check for gcc version: support for noexcept on class constructor a...

fixed check for gcc version: support for noexcept on class constructor available only since gcc 4.6( https://gcc.gnu.org/projects/cxx0x.html )

for example it's used here https://github.com/reverbrain/elliptics/blob/ebd6268aca16f808e5884121059b637f90a3a247/library/backend.h#L72