security/afl: Update to 2.39b

[tobik]

Summary

Update to 2.39b

- Move to LLVM 3.9
- Remove TEST_INSTRUMENTATION option and move instrumentation tests to
  the test stage
- Remove MINIMIZE_CORPUS option: afl-cmin was always installed anyway
- Remove useless pkg-message: afl-cplot prompts you to install gnuplot
  on its own

Changes: http://lcamtuf.coredump.cx/afl/ChangeLog.txt
Approved by:	??? (mentor)
Differential Revision:	https://reviews.freebsd.org/D????

Test Plan

poudriere testport 10.3/i386 ok
poudriere testport 11.0/amd64 ok