Added ability to detect and use either gcc or Sun WorkShop C compiler

Added ability to detect and use either gcc or Sun WorkShop C compiler
on Solaris.  Added support for Solaris 10.  Quieted down warning in
ppp_comp.c due to bad preprocessor usage.  Quieted WorkShop warnings
in options.c (casting of void * to function) and pppd.h (constant too
large).  Tested in 32 and 64 bit modes with gcc and WorkShop.