RFR: JDK-8210723 Better information in configure for invalid Xcode

[Magnus Ihse Bursie]

When running e.g. Xcode 9.4 on macosx 10.12, the configure scripts fails 
with: "Failed to determine Xcode version" This is not very helpful.

Adding the output of the failed xcodebuild call (typically something 
like "Executable requires at least macOS 10.13, but is being run on 
macOS 10.12.6, and so is exiting") will help the user diagnose the error.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210723
Patch inline:
diff --git a/make/autoconf/toolchain.m4 b/make/autoconf/toolchain.m4
--- a/make/autoconf/toolchain.m4
+++ b/make/autoconf/toolchain.m4
@@ -232,6 +232,7 @@
        XCODE_VERSION_OUTPUT=`"$XCODEBUILD" -version 2>&1 | $HEAD -n 1`
        $ECHO "$XCODE_VERSION_OUTPUT" | $GREP "Xcode " > /dev/null
        if test $? -ne 0; then
+        AC_MSG_NOTICE([xcodebuild output: $XCODE_VERSION_OUTPUT])
          AC_MSG_ERROR([Failed to determine Xcode version.])
        fi
        XCODE_MAJOR_VERSION=`$ECHO $XCODE_VERSION_OUTPUT | \

/Magnus