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

[Erik Joelsson]

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/

/Erik