Add build environment for FreeBSD

Tested on FreeBSD 9.1 (i386) with both clang and gcc

  FreeBSD clang version 3.1 (branches/release_31 156863) 20120523
  gcc version 4.2.1 20070831 patched [FreeBSD]

And FreeBSD 8.4 (x64) with gcc

  gcc version 4.2.1 20070831 patched [FreeBSD]
Add build environment for DragonflyBSD

Tested on DragonflyBSD v3.4.2-RELEASE (x64) with gcc

  gcc version 4.7.2 [DragonFly] Release/2012-09-20
Add build environment for NetBSD

Tested on NetBSD 6.1 (x64) with gcc

  gcc version 4.5.3 (NetBSD nb2 20110806)