You might want to add the following to the Makefile:

# for IRIX 6.2
CFLAGS = -fullwarn -ansi -O2

# for IRIX 6.3 & up
CFLAGS = -fullwarn -ansi -O3

# for all IRIX, if using R4K, add:
CFLAGS += -mips3

# for all IRIX, if using R5K/R8K/R10K, add:
CFLAGS += -mips4

# for all IRIX, can also try using (see cc(1) for details on how to specify
# the processor) just the following (i.e., do not use any other flags):
CFLAGS = -fullwarn -ansi -Ofast=ipxx

Many thanks to Terry for these hints.

------------------------------------------------------------------------------