Stack traces were not printed by our signal handler on FreeBSD
since we didn't find backtrace(). However, since FreeBSD 10.0
backtrace() is available, but it requires the execinfo library.

This patch fixes the problem by adding -lexecinfo when
doing CMake configure checks and when linking the server
(if execinfo exists). Note that the number of stack frames
returned by backtrace() seems very limited - at least on
FreeBSD 10.2.

Also, the PTR_SANE macro which tries to check if a pointer
is invalid (used when printing pointer values in stack traces)
gave false negatives on OSX/FreeBSD. On these platforms we
now simply check if the pointer is non-null. This also removes
a sbrk() deprecation warning when building on OS X. (It was
before only disabled with building using XCode).