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.