Try to find a compiler option that warns when a function prototype
is not fully defined. Enable it only if the the system headers
are reasonably clean with respect to compiling with strict-prototypes.
The sanity check is done by looking at sys/signal.h which has a set
of macro-definitions SIG_DFL and SIG_IGN that are cast to the local
signal-handler type. If that signal-handler type is not fully
qualified then the system headers are not seen as strictly prototype clean.
Currently this macro knows about GCC.
hopefully will evolve to use: Solaris C compiler,
Digital Unix C compiler, C for AIX Compiler, HP-UX C compiler,
and IRIX C compiler.