RFR: JDK-8201508: Macosx builds fail in GenerateLinkOptData.gmk

[Tim Bell]

Erik:

> All macosx builds in CI currently fail. This is caused by in issue with
> JDK-8201483 which leaves the JVM_VARIANTS_server variable empty. The
> construct used to subtract one set of strings from another doesn't work
> quite as expected with bsd grep.
>
> result=`$GREP -Fvx "$legal_values" <<< "$values_to_check" | $GREP -v '^$'`
>
> If the set to subtract ($legal_values) is empty, gnu grep returns all
> values (as expected) while bsd grep returns nothing. To fix this I've
> added an explicit check for the base case of legal_values being empty.
> This seems to make --with-jvm-features work correctly on macosx and
> still work correctly on Linux. A verification distributed build is under
> way.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8201508
>
> Webrev: http://cr.openjdk.java.net/~erikj/8201508/webrev.01/

Looks good.

Tim