This fixes the "does unsetenv return void" test in the unix package on
OS X, if I tell it to make 10.4-compatible binaries. The test uses
CPPFLAGS but not CFLAGS, so it thought it returned int (as it was
in 10.5-mode), but the C compiler (using CFLAGS, so in 10.4 mode)
thought it returned void.

I also added CONF_LD_OPTS_STAGE$3 to the list of things in LDFLAGS,
which looks like an accidental ommission.