lang/gcc6-aux: Update to 20170802 snapshot [1]

• Make sure gmake can be found during the build when PREFIX is different from LOCALBASE [1]
• This also fixes fetching the port since the 20170202 snapshot is gone from a lot of mirrors [2]

PR: 221883 [1, 2], 222133 [2]
Submitted by: frederic.praca@free.fr (based on) [1]
Reported by: mayhem30@gmail.com [2]
MFH: 2017Q4