Fetching third-party files inside a make rule is convenient and less
error prone since 'make' has access to '$srcdir'.
* build-aux/gnu-fetch: Delete.
* maint.mk (WGET, git_sv_host, sv_git_am, sv_git_gl)
(fetchfiles): New macros.
(fetch): New target.
* HACKING <Updating auxilary scripts>: Update instructions.