pgsql: When using GCC, change the default CFLAGS to: -O2 -Wall

Log Message:
-----------
When using GCC, change the default CFLAGS to:

-O2 -Wall -Wmissing-prototypes -Wpointer-arith

Check whether the version of GCC we are using supports any of:

-Wdeclaration-after-statement
-Wendif-labels
-Wold-style-definition

And add the supported flags to CFLAGS.

Modified Files:
--------------
pgsql:
configure (r1.402 -> r1.403)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.402&r2=1.403)
configure.in (r1.381 -> r1.382)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.381&r2=1.382)
pgsql/config:
c-compiler.m4 (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/config/c-compiler.m4.diff?r1=1.12&r2=1.13)
pgsql/src:
Makefile.global.in (r1.203 -> r1.204)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.global.in.diff?r1=1.203&r2=1.204)