build.sh: 2nd refactoring pt. I.
    build/{gcc,musl}.*.build: refactored into vars/{gcc,musl}.*.build.
    build/pkg.build: merged w/ build.sh (structurally) and refactored into subr/pkg_*.subr (implementations.)
    patches/gcc_native_x86_64_w64_mingw32.local.patch: fix wrong filename.
    subr/build.subr: merges source_scripts() into build.sh.
    subr/pkg.subr: accept either of ${PKG_..._{URL,VERSION}} when checking for missing entries in var/build.vars.
    vars/build.vars: adds vars for {gcc,musl}_*.
    vars/psxstub.vars: convert to pkg_..._all(); call pkg_fetch().