Additional minor FreeBSD fixes

These are the final two changes I needed to get liblldb built on FreeBSD.

Index: CMakeLists.txt

And one change to get lldb to link. FreeBSD's libc doesn't have
backtrace() et al., but it's available in libexecinfo (in the FreeBSD
ports tree in devel/libexecinfo).

At this point (svn r183221 and the two changes in this thread) 37 out
of 253 tests pass. In almost all cases the failures are due to the
test Makefiles requiring GNU make, which is installed as 'gmake' on
FreeBSD. Perhaps I'll add a getMake() function to to
detect the make spelling to use.

Index: CMakeLists.txt

Thanks Ed! I committed your build fixes in 183233.

Your approach to fix the tests sounds reasonable, go for it!