Add support for gcc 4.9

GCC errors and warnings are colorized when -fdiagnostics-color=auto or
-fdiagnostics-color=always specified. With the existence of the
environment variable GCC_COLORS, the default value for
-fdiagnostics-color is auto, or never otherwise, so I explicitly assign
the value of GCC_COLORS in the shell initialization scrips. Here the
value is copied from the man page of gcc.