RFR: JDK-8033292 configure MAKE=foo causes configure to fail

[Magnus Ihse Bursie]

Bug: https://bugs.openjdk.java.net/browse/JDK-8033292

With the fix for JDK-8031759, I added a check for unknown variables 
given on the configure command line.

Unfortunately, this check required that all known variables used by 
configure "register" themselves; if this is not done, then the variable 
is considered unknown. And not all cases of such variable usages was 
checked by me at that time. :-(

A prominent example is MAKE.

While I think the way forward is to properly register all such uses (and 
probably unify tool detection even more across the line), at the moment, 
this check should be a warning, not an error.

This fix is too small for a webrev; please see inlined patch.

Patch:
diff --git a/common/autoconf/basics.m4 b/common/autoconf/basics.m4
--- a/common/autoconf/basics.m4
+++ b/common/autoconf/basics.m4
@@ -250,7 +250,7 @@
    if test "x$CONFIGURE_OVERRIDDEN_VARIABLES" != x; then
      # Replace the separating ! with spaces before presenting for end user.
      unknown_variables=${CONFIGURE_OVERRIDDEN_VARIABLES//!/ }
-    AC_MSG_ERROR([The following variables are unknown to configure: 
$unknown_variables])
+    AC_MSG_WARN([The following variables might be unknown to configure: 
$unknown_variables])
    fi
  ])


/Magnus