Add FindExecinfo.cmake

CMake file to help find the execinfo includes and library. The problem with this library is that it is built-in in the Linux glib, while on systems like FreeBSD, it is installed separately and thus needs to be linked to. Therefore, we search for the header to see if the it's available in the first place. If it is available, we try to locate the library to figure out whether it is built-in or not.