pregen: Fix the parallel build problem in pregen target

* pregen-headers really need the headers to get themselves generated.
  so we create explicit dependency and invoke make as an action on
  pregen. This fixes the parallel build issue for better.

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

Makefile.in

@@ -16,9 +16,6 @@ noconfig_targets := menuconfig config oldconfig silentoldconfig randconfig \
 
 include $(top_srcdir)Rules.mak
 sub_headers := headers
-
-pregen-headers-y := $(top_builddir)include/bits/sysnum.h
-
 ifndef KCONFIG_CONFIG
 KCONFIG_CONFIG := $(top_builddir).config
 endif
@@ -160,9 +157,10 @@ headers_clean-y += sysdep_common_headers-clean
 # libc/sysdeps/linux/Makefile.commonarch to headers-y
 headers-y += $(target-headers-sysdep)
 
-headers: $(top_builddir)include/bits/uClibc_config.h
+headers: $(top_builddir)include/bits/uClibc_config.h $(top_builddir)include/bits/sysnum.h
 subdirs: $(addprefix $(top_builddir),$(subdirs))
-pregen: headers $(pregen-headers-y) subdirs
+pregen: headers subdirs
+	$(Q)$(MAKE) $(pregen-headers-y)
 	$(Q)$(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