Fix simtest build on MacOS X.

On mac stdlib.h includes signal.h which conflicts with the
variable of the same name in this file. Unfortunately that
makes it something of a reserved identifier in portable
code, but the only stdlib.h call this file actually uses
is exit(). Returning from main() is equivalent, so we can
just remove the stdlib.h include instead.
Update older codes to use limits.h.

This is more portable that limux/limits.h and the unprefixed
version is older than the fftw library we depend on.
Fix icesync compilation on MacOS X.

I don't know if the trailing 'l' here is a typo, or intended
as a length marker on the double constant, but apple clang 5.1
interprets it as part of the macro name and fails to compile.

The macro is defined with sufficient precision for use as a double
here.
Remove commented out include.

Found this converting the other linux/limits.h to limits.h.