Currently this is Linux only as it requires the Linux implementation of
POSIX signals.

There is a simple API, and the profile output is written to
/tmp/gjs-profile-<pid>. Description of the capture format is provided
in profile.cpp.

For apps started with gjs-console, you can toggle on/off profiling by
sending SIGUSR2 to the process.

(Later modified by Philip Chimento; trimmed unneeded functionality out of
the sysprof capture code, fixed coding style, fixed crash by unsetting
the profiling stack on stop, and switched the license to conform to the
rest of the project.)

Closes #31.