Fix capitalization and make it build on FreeBSD 9-10.
Allow using builtin libexecinfo on FreeBSD:
FreeBSD has started using NetBSD's libexecinfo since version 10.
Treat NetBSD, FreeBSD, and OpenBSD uniformly.
This fixes build on FreeBSD 10 (tested by Youri Mouton) and 9.1.
Use GCC_REQD+=4.6, package needs C++11 features for kactivitymanagerd
otherwise it won't be built and there are PLIST failures.