build: drop extra qbconfig.h rule for auto_check_header self-test

Although this is not as rigorously correct as the previous arrangement
(introduced with 80834ea), i.e., from the perspective of
$(abs_top_builddir)/tests/Makefile (generated from from
$(top_srcdir)/tests/Makefile.am), for which $(top_builddir)
is literally "..":

- qbconfig.h is generated from $(top_srcdir)/include/qb/qbconfig.h.in
  into $(top_builddir)/include/qb directory, and consumed from here

- rest of these header files are consumed directly from
  $(top_srcdir)/include/qb directory

this new simplified variant:

- avoids code duplication (DRY)

- works thanks to VPATH in $(abs_top_builddir)/tests/Makefile
  pointing to $(top_srcdir)/tests, which means that unsatisfied
  dependencies on "$(top_builddir)/include/qb/%.h" (resolved to
  "../include/qb/%.h") will be subsequently sought as
  "$(top_srcdir)/tests/../include/qb/%.h", which has the same effect
  as previous explicit "$(top_srcdir)/include/qb/%.h" specification
  (note that such automatic VPATH provision is supported by automake
   "since beginning" so no backward incompatibility is expected)

tests/Makefile.am

@@ -56,21 +56,11 @@ auto_c_files = $(patsubst %.h,auto_check_header_%.c,$(public_headers))
 CLEANFILES += $(auto_c_files)
 MAINTAINERCLEANFILES += $(auto_c_files)
 
-#
-# The qbconfig.h file is generated from qbconfig.h.in into the build directory tree.
-# We have to have a separate rule for generating the auto_check_header_ source here.
-#
-$(builddir)/auto_check_header_qbconfig.c: $(top_builddir)/include/qb/qbconfig.h
-	@name=$$(echo "$<" | sed  "s|.*qb/qb||" | sed "s|\.h||") ;\
-	NAME=$$(echo $$name | tr [:lower:] [:upper:]) ;\
-	echo "#include <qb/qb$$name.h>" > $@_ ;\
-	echo "#ifndef QB_$${NAME}_H_DEFINED" >> $@_ ;\
-	echo "#error no header protector in file qb$$name.h" >> $@_ ;\
-	echo "#endif" >> $@_ ;\
-	echo "int main(void) {return 0;}" >> $@_
-	$(AM_V_GEN)mv $@_ $@
-
-$(builddir)/auto_check_header_%.c: $(top_srcdir)/include/qb/%.h
+# this works for both non/generated headers thanks to VPATH being
+# automatically set to $(top_srcdir)/tests and $(top_builddir)
+# being resolved to ".." by automake
+# ($(top_srcdir)/tests/../include/qb/%.h = $(top_srcdir)/include/qb/%.h)
+$(builddir)/auto_check_header_%.c: $(top_builddir)/include/qb/%.h
 	@name=$$(echo "$<" | sed  "s|.*qb/qb||" | sed "s|\.h||") ;\
 	NAME=$$(echo $$name | tr [:lower:] [:upper:]) ;\
 	echo "#include <qb/qb$$name.h>" > $@_ ;\