Log Message:
Nuke USE_FORTRAN and bring the f2c handling within the mk/compiler
framework.  The list of changes include:

  * Modify compiler.mk so that "c" is always prepended to USE_LANGUAGES,
    so we no longer need to say it in package Makefiles.  Packages
    should now append to USE_LANGUAGES instead of setting it.

  * Create mk/compiler/f2c.mk which implements another pseudo-compiler
    "f2c" that may be used with any C compiler backend, e.g.

	PKGSRC_COMPILER= f2c ccache gcc

  * Teach the various "real" compiler files, e.g., sunpro.mk, mipspro.mk,
    etc., to use f2c if the native Fortran compiler isn't present.

Packages that use Fortran should now simply include the line:

	USE_LANGUAGES+=	fortran

in the package Makefile.