* guix/build-system/gnu.scm (%default-modules): Rename to...
(%gnu-build-system-modules): ... this.
(%default-modules): New variable.
(dist-package, gnu-build): Use %GNU-BUILD-SYSTEM-MODULES for
#:imported-modules.
(gnu-cross-build): Likewise, and use %DEFAULT-MODULES for #:modules.
* guix/build-system/cmake.scm (%cmake-build-system-modules): New
variable.
(cmake-build): Use it for #:imported-modules.
* guix/build-system/glib-or-gtk.scm (%default-imported-modules): Rename
to...
(%glib-or-gtk-build-system-modules): ... this. Refer to
%GNU-BUILD-SYSTEM-MODULES. Adjust uses.
* guix/build-system/perl.scm (%perl-build-system-modules): New
variable.
(perl-build): Use it for #:imported-modules.
* guix/build-system/python.scm (%python-build-system-modules): New
variable.
(python-build): Use it for #:imported-modules.
* guix/build-system/ruby.scm (%ruby-build-system-modules): New variable.
(ruby-build): Use it for #:imported-modules.
* guix/build-system/waf.scm (%waf-build-system-modules): New variable.
(waf-build): Use it for #:imported-modules.