normalize values of DO_FETCH_TARBALLS

This one has additional value "ALL".

Change-Id: Id804172eee899b443a6f8ae6bce8781136096dc7

Makefile.in

@@ -174,7 +174,7 @@ cmd:
 #
 # Fetch
 #
-ifneq ($(DO_FETCH_TARBALLS),NO)
+ifneq ($(DO_FETCH_TARBALLS),)
 include $(SRCDIR)/Makefile.fetch
 fetch: download
 fetch: get-submodules

configure.ac

@@ -4785,11 +4785,11 @@ if test "$enable_fetch_external" != "no"; then
         DO_FETCH_TARBALLS="ALL"
     else
         AC_MSG_RESULT(["yes, if we use them"])
-        DO_FETCH_TARBALLS="YES"
+        DO_FETCH_TARBALLS="TRUE"
     fi
 else
     AC_MSG_RESULT([no])
-    DO_FETCH_TARBALLS="NO"
+    DO_FETCH_TARBALLS=
 fi
 AC_SUBST(DO_FETCH_TARBALLS)