maint: teach "make syntax-check" the space-only indentation rule

* cfg.mk (sc_prohibit_tab_based_indentation): New rule.
* .x-sc_prohibit_tab_based_indentation: New file.
* Makefile.am (syntax_check_exceptions): Add file,
.x-sc_prohibit_tab_based_indentation.

.x-sc_prohibit_tab_based_indentation

@@ -0,0 +1,6 @@
+^GNUMakefile$
+Makefile\.am$
+\.mk$
+^tests/pr/
+ChangeLog.*
+^man/help2man$

Makefile.am

@@ -49,6 +49,7 @@ syntax_check_exceptions =		\
   .x-sc_prohibit_atoi_atof		\
   .x-sc_prohibit_stat_st_blocks		\
   .x-sc_prohibit_strcmp			\
+  .x-sc_prohibit_tab_based_indentation	\
   .x-sc_require_config_h		\
   .x-sc_require_config_h_first		\
   .x-sc_space_tab			\

cfg.mk

@@ -206,4 +206,10 @@ sc_strftime_check:
 	  rm -f $@-src $@-info;						\
 	fi
 
+# Indent only with spaces.
+sc_prohibit_tab_based_indentation:
+	@re='^ *	'						\
+	msg='TAB in indentation; use only spaces'			\
+	  $(_prohibit_regexp)
+
 include $(srcdir)/dist-check.mk