Overhauling gcc.mk

[Jonathan Perkin <jperkin%mnx.io@localhost>]

As promised, I've been completely renovating mk/compiler/gcc.mk to 
hopefully reduce a lot of the complexity, make it less likely that 
future changes will result in accidental breakage, add numerous 
comments, as well as improving performance.

The branch is here:

  https://github.com/NetBSD/pkgsrc/compare/trunk...TritonDataCenter:pkgsrc:feature/gcc-selection/trunk

Overview:

  * Move GCC_REQD and associated logic into a separate file.

  * If the user has requested USE_NATIVE_GCC=yes then skip all of the
    GCC_REQD processing.

  * Add support for GCC_BOOTSTRAP_PKGS, a list of PKGPATH packages that
    are in the dependency path for gcc and gcc-libs, making it easier
    for users to avoid circular dependencies when USE_PKGSRC_GCC=yes.

  * Make it easier for users to select their own GCC packages, e.g.
    from wip or other repositories.

  * Remove _USE_GCC_SHLIB and USE_GCC_RUNTIME, and simply enable based
    on USE_PKGSRC_GCC_RUNTIME, with the same exclusions as above.
    Removing USE_GCC_RUNTIME from individual packages can be done later.

  * Remove ALLOW_NEWER_COMPILER, it essentially duplicates the
    USE_NATIVE_GCC logic and confuses things.

  * Remove bogus _LANGUAGES setup, assume GCC packages support all
    languages, and simplify.

  * Simplify GCC version detection, reducing number of forks, and remove
    support for long-obsolete compilers.

  * Simplify NetBSD USE_PKGSRC_GCC_RUNTIME selection.

  * Rename a bunch of variables to make it clearer what they actually
    do.

  * Use newer make syntax.

  * Stop trying to support NetBSD < 1.5.

This will obviously require lots of testing before it even goes near the 
tree, but I would appreciate feedback on any of the more contentious 
parts, and obviously testing would be fantastic too.

Thanks,

--
Jonathan Perkin   -   mnx.io   -   pkgsrc.smartos.org
Open Source Complete Cloud   www.tritondatacenter.com