build: grab "dependent_headers" from respective Makefile.am

...with "echo path/*.h" fallback (equivalent of wildcard function) if
the new methods fails for some reason.

docs/Makefile.am

@@ -24,14 +24,19 @@ noinst_HEADERS          = mainpage.h
 dist_man_MANS = man8/qb-blackbox.8
 if HAVE_DOXYGEN
 inc_dir = $(top_srcdir)/include/qb
-dependent_headers = $(wildcard $(inc_dir)/qb*.h)
+
+dependent_headers = $(subst $(inc_dir),,$(shell \
+	printf 'include $(inc_dir)/Makefile.am\n\n%%.var:\n\t@echo $$($$*)' \
+	| ${MAKE} --no-print-directory -f - inst_HEADERS.var \
+	|| echo $(inc_dir)/qb*.h))
+
 dist_man_MANS += man3/qbipcc.h.3 man3/qbipcs.h.3 man3/qbatomic.h.3 \
 		 man3/qbhdb.h.3 man3/qbipc_common.h.3 man3/qblist.h.3 \
 		 man3/qbloop.h.3 man3/qbutil.h.3 man3/qbarray.h.3 \
 		 man3/qblog.h.3 man3/qbmap.h.3
 
 
-$(dist_man_MANS): man.dox $(dependent_headers)
+$(dist_man_MANS): man.dox $(dependent_headers:%=$(inc_dir)/%)
 	mkdir -p man3
 	doxygen man.dox