additional GCC warning flags

This patch adds code to "configure" to check if GCC supports the
following warning flags: -Wdeclaration-after-statement (GCC 3.4+),
-Wold-style-definition (GCC 3.4+), and -Wendif-labels (GCC 3.3+). Any of
these options that are supported by $CC are added to $CFLAGS. The patch
also removes -Wmissing-declarations from the default CFLAGS, on the
grounds that it is (I believe) redundant when -Wmissing-prototypes is
also specified.

(I didn't mention -Wendif-labels in my mail to -hackers, but it should
be pretty innocuous: "Warn whenever an #else or an #endif are followed
by text.")

This patch updates configure.in and configure (I re-ran autoconf 2.53).
It doesn't introduce any additional warning messages locally (Linux, GCC
3.4), but might do so on some platforms (the code in src/port/ is likely
to trigger some warnings, I think). Because of that, I think it is
probably best to save this for 8.1

-Neil