Make CPPFLAGS variables, as well as CFLAGS and LDFLAGS

Make CPPFLAGS variables, as well as CFLAGS and LDFLAGS
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.