build: allow whitespace violations in gl/lib/*.diff files

* .gitattributes: Exempt gl/lib/*.diff.
* .x-sc_prohibit_tab_based_indentation: Likewise.
* .x-sc_space_tab:Likewise.

.gitattributes

@@ -5,3 +5,5 @@
 # # Derived from the regexp in emacs' lisp/add-log.el.
 # [diff "texinfo"]
 #         funcname = "^@node[ \t][ \t]*\\([^,][^,]*\\)"
+
+gl/lib/*.diff -whitespace

.x-sc_prohibit_tab_based_indentation

@@ -4,3 +4,4 @@ Makefile\.am$
 ^tests/pr/
 ChangeLog.*
 ^man/help2man$
+^gl/lib/.*\.c\.diff$

.x-sc_space_tab

@@ -9,3 +9,4 @@ m4/lib-prefix.m4
 m4/po.m4
 aclocal.m4
 src/c99-to-c89.diff
+^gl/lib/.*\.c\.diff$