We have two very new machines running Solaris 2.4.
I had not got round to compiling GAP on them until today.
(One reason for this is that GAP compiled on SUN 4 using
sun-sparc-sunos-cc does run on the Solaris machines (although this
compatability does not seem to work in the other direction).

Unfortunately, we also do not have cc (since SUN no longer supply it
free) so we have to use gcc. I eventually got it to compile and run
successfully with

make usg CC=gcc COPTS="-DSYS_HAS_SIGNAL_PROTO -DSYS_HAS_TIME_PROTO"

Best wishes,
Derek.