maint: refresh stale local gnulib patch files

We carry local adjustments for a few gnulib modules via the
patches in gl/.  Nearly all of those patches had become stale
due to evolution of the originals in gnulib.

To refresh them, first make sure you have no local changes in gl/
or in the gnulib submodule, then run "make refresh-gnulib-patches".

gl/modules/link-tests.diff

@@ -1,8 +1,8 @@
-diff --git i/modules/link-tests w/modules/link-tests
-index d8e7b1a..aca0e74 100644
---- i/modules/link-tests
-+++ w/modules/link-tests
-@@ -12,3 +12,4 @@ configure.ac:
+diff --git a/modules/link-tests b/modules/link-tests
+index 5b1978e..039e97f 100644
+--- a/modules/link-tests
++++ b/modules/link-tests
+@@ -15,3 +15,4 @@ configure.ac:
  Makefile.am:
  TESTS += test-link
  check_PROGRAMS += test-link