Fix mod_mongo build when no CFLAGS/CXXFLAGS are set in environment

os.environ['CFLAGS'] raises a KeyError if no CFLAGS variable is set in
the environment.  So instead we use os.environ.get('CFLAGS','') which
will return the empty string in the event no CFLAGS variable is set.

The previous problematic pattern originated from the suggested code
here for importing the build environment in SCons:

  http://www.scons.org/wiki/FrequentlyAskedQuestions#Why_doesn.27t_SCons_find_my_compiler.2BAC8-linker.2BAC8-etc..3F_I_can_execute_it_just_fine_from_the_command_line.

FS-4746 --resolve