--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2010-04-27  Jim Meyering  <meyering@redhat.com>
+
+	maint.mk: syntax-check: prohibit HAVE_<header>_H that are always true
+	Derive the list of guaranteed header names from gnulib/lib/*.in.h,
+	and use that to create a regexp used to detect all #if HAVE_..._H uses.
+	* top/maint.mk (sc_prohibit_always_true_header_tests): New rule.
+	(gl_assured_headers_, az_, AZ_): Define.
+	(gl_header_upper_case_or_, gl_have_header_regex_): Define.
+
 2010-04-26  Jim Meyering  <jim@meyering.net>
 	    Bruno Haible  <bruno@clisp.org>
 

--- a/top/maint.mk
+++ b/top/maint.mk
@@ -645,6 +645,28 @@
 	halt='found useless parentheses in cpp directive'		\
 	  $(_sc_search_regexp)
 
+# List headers for which HAVE_HEADER_H is always true, assuming you are
+# using the appropriate gnulib module.  CAUTION: for each "unnecessary"
+# #if HAVE_HEADER_H that you remove, be sure that your project explicitly
+# requires the gnulib module that guarantees the usability of that header.
+gl_assured_headers_ := \
+  $(shell cd $(gnulib_dir)/lib && ls -1 *.in.h|sed 's/\.in\.h$$/ \\/')
+
+# Convert the list of names to upper case, and replace each space with "|".
+az_ = abcdefghijklmnopqrstuvwxyz
+AZ_ = ABCDEFGHIJKLMNOPQRSTUVWXYZ
+gl_header_upper_case_or_ := \
+  $(shell echo $(gl_assured_headers_)					\
+    | tr $(az_)/.- $(AZ_)___						\
+    | tr -s ' ' '|'							\
+   )
+gl_have_header_regex_ = HAVE_($(gl_header_upper_case_or_))_H
+sc_prohibit_always_true_header_tests:
+	@prohibit='\<$(gl_have_header_regex_)\>'			\
+	halt='do not test the above HAVE_<header>_H symbol(s);\n'\
+'  with the corresponding gnulib module, they are always true'		\
+	  $(_sc_search_regexp)
+
 # Prohibit checked in backup files.
 sc_prohibit_backup_files:
 	@$(VC_LIST) | grep '~$$' &&				\