buildsys: Tweak pregen wrt headers_dep

Required for !NPTL, !context-funcs for example.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Makefile.in

@@ -186,9 +186,7 @@ headers: $(top_builddir)include/bits/uClibc_config.h | subdirs
 subdirs: $(addprefix $(top_builddir),$(subdirs))
 
 $(pregen-headers-y): $(headers_dep)
-
-pregen-headers: $(pregen-headers-y)
-pregen: headers pregen-headers
+pregen: headers $(pregen-headers-y) $(headers_dep)
 	$(Q)$(if $(UCLIBC_HAS_LOCALE),$(MAKE) -C extra/locale locale_headers)
 
 $(top_builddir)include/bits/sysnum.h: $(top_srcdir)extra/scripts/gen_bits_syscall_h.sh | $(top_builddir)include/bits