[NETBEANS-3209] Add JAVA_HOME and ANT_HOME note

Add a brief note about the JAVA_HOME and ANT_HOME environment variables
to the build instructions on the Web site and the GitHub repository. See
the following message on the mailing list for details:

Re: Building NetBeans 11.1 from source fails with compile errors
https://mail-archives.apache.org/mod_mbox/netbeans-dev/201910.mbox/%3C26c1f141-b021-4473-059f-41ec0b5e3613%40status6.com%3E

Fixes #3209
1 file changed