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.
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