Minor configure tweak to simplify adjusting gcc warnings

Would anyone object to modifying configure.in like this:

if test "$GCC" = yes -a "$ICC" = no; then
- CFLAGS="$CFLAGS -Wall -Wmissing-prototypes -Wpointer-arith"
+ CFLAGS="-Wall $CFLAGS -Wmissing-prototypes -Wpointer-arith"
# These work in some but not all gcc versions
PGAC_PROG_CC_CFLAGS_OPT([-Wdeclaration-after-statement])

The reason I got interested in this is that I attempted to pass in
"CFLAGS=-Wno-format" to configure, to suppress format warnings on
buildfarm member gaur (whose gcc is too old to recognize z modifiers).
That doesn't work because -Wall turns the warnings right back on again.
If the user-supplied CFLAGS were inserted after -Wall then it would work.

A slightly more complicated change could be applied to make sure that
*all* of the CFLAGS forcibly inserted by configure appear before any
externally-sourced CFLAGS, allowing any of them to be overridden from the
environment variable. I'm not sure if it's worth the trouble to do that,
but if there's interest I could make it happen.

regards, tom lane