hurd: Break errnos.d / libc-modules.h dependency loop

Generating errnos.d does not actually need libc-modules.h.

* sysdeps/mach/hurd/Makefile ($(common-objpfx)errnos.d): Strip
"-include $(common-objpfx)libc-modules.h" from CPPFLAGS, and do not
depend on libc-modules.h,

ChangeLog

@@ -1,3 +1,9 @@
+2016-03-20  Samuel Thibault  <samuel.thibault@ens-lyon.org>
+
+	* sysdeps/mach/hurd/Makefile ($(common-objpfx)errnos.d): Strip
+	"-include $(common-objpfx)libc-modules.h" from CPPFLAGS, and do not
+	depend on libc-modules.h,
+
 2016-03-17  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_EVENTFD2):

sysdeps/mach/hurd/Makefile

@@ -79,9 +79,11 @@ endef
 # the headers we want to search for Mach error codes, listed above (and
 # incidentally, all other headers those include).
 -include $(common-objpfx)errnos.d
-$(common-objpfx)errnos.d: $(mach-errnos-deps) libc-modules.h
+$(common-objpfx)errnos.d: $(mach-errnos-deps)
 	$(mach-errno-h) | \
-	$(CC) $(CFLAGS) $(CPPFLAGS) -M -x c - | \
+	$(CC) $(CFLAGS) \
+	    $(subst -include $(common-objpfx)libc-modules.h,,$(CPPFLAGS)) \
+	    -M -x c - | \
 	sed $(sed-remove-objpfx) -e 's,- *:,mach-errnos-deps :=,' \
 	    -e 's, \.\./, $(..),g' > $@t
 	mv -f $@t $@