pregen-headers: Add new target which depends on headers.

* This target is necessary to serialize making of
  headers before pregen-headers and pregen-headers
  before the rest of build

Signed-off-by: Khem Raj <raj.khem@gmail.com>

Makefile.in

@@ -159,10 +159,10 @@ headers-y += $(target-headers-sysdep)
 
 headers: $(top_builddir)include/bits/uClibc_config.h $(top_builddir)include/bits/sysnum.h
 subdirs: $(addprefix $(top_builddir),$(subdirs))
-pregen: headers subdirs
+pregen-headers: headers
 	$(Q)$(MAKE) $(pregen-headers-y)
+pregen: pregen-headers subdirs
 	$(Q)$(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
 	$(Q)$(INSTALL) -d $(@D)
 	@$(disp_gen)