[patch, fortran] Set -Wcompare-reals from -Wextra

[Thomas Koenig]

Hello world,

the attached patch sets -Wcompare-reals from -Wextra. It also
dcouments a few cases (found while browsing the source) of
options included in -Wall in invoke.texi.  It also allows easy
adding of other warning options to -Wextra.

Regression-tested. OK for trunk?

	Thomas

2012-09-01  Thomas König  <tkoenig@gcc.gnu.org>

         * lang.opt (Wextra):  Add.
         * invoke.texi:  Document that -Wc-binding-type, -Wconversion
         and -Wline-truncation are implied by -Wall.  Document that
         -Wcompare-reals is implied by -Wextra.
         * options.c (set_Wextra):  New function.
         (gfc_handle_option):  Handle -Wextra.

2012-09-01  Thomas König  <tkoenig@gcc.gnu.org>

         * gfortran.dg/wextra_1.f:  New test.