From the GCC 4.9 release notes for Fortran:

 https://gcc.gnu.org/gcc-4.9/changes.html
 "Module files: The version of the module files (.mod) has been
  incremented; additionally, module files are now compressed."

Teach cmDependsFortran::ModulesDiffer to look for the gzip magic numbers
at the beginning of the module file.  If found, assume the module was
produced by gfortran >= 4.9.  The modules do not appear to contain the
date as earlier versions did so we can compare the content directly
and do not actually need to decompress.