FreeBSD support of build infrastructure

This adds FreeBSD support to osquery's build system, and only does it.  It could be the preliminary work of porting osquery to FreeBSD, and enables more people start to work on it without solving tedious build issues.

Build process:

1) Install dependencies:

autoconf
automake
bison
boost
cmake
gflags
git
glog
gmake
libtool
libunwind
rocksdb
snappy
thrift
thrift-cpp

tools/provision.sh did not install build packages for FreeBSD, just skips them.

2)
For now, the implementations of following tables do not exist (yet):

osquery/tables/specs/x/groups.table
osquery/tables/specs/x/passwd_changes.table
osquery/tables/specs/x/process_envs.table
osquery/tables/specs/x/process_open_files.table
osquery/tables/specs/x/processes.table
osquery/tables/specs/x/routes.table
osquery/tables/specs/x/users.table

By removing them temporarily, osquery can be built under FreeBSD with just `gmake`.
So you recommend finishing the dependency installation part before merging?  Fair enough.  I submit this pull request before having tables under `spec/x` finished just because I found we have some clean up in the build infrastructure last week, which made some conflicts and I don't want to go through the build scripts 3rd times :)