Bogosity in contrib/xml2/Makefile

Whilst fooling with bug #4058 I noticed that xml2's .c files were being
compiled without -g or any of the various warning flags we normally use.
I saw this:

gcc -I/usr/include/libxml2 -fpic -I. -I../../src/include -D_GNU_SOURCE -I/usr/include/libxml2   -c -o xpath.o xpath.c

when I expected something like this:

gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Winline -Wdeclaration-after-statement -Wendif-labels -fno-strict-aliasing -fwrapv -g -fpic -I. -I../../src/include -D_GNU_SOURCE -I/usr/include/libxml2   -c -o xpath.o xpath.c

The reason is apparently this line in its Makefile:

override CFLAGS += $(shell xml2-config --cflags)

It seems the "override" locks down the value so that the subsequent
assignment in Makefile.global does nothing.  I didn't try the PGXS
case but I imagine it doesn't do the right thing either.

Now, in HEAD and 8.3 I think we could just remove this line, because
configure knows how to pull the needed -I and -L flags out of
xml2-config's output and stick them into appropriate flag variables
(neither of which is CFLAGS btw...).  I am not sure what to do in older
branches though --- there doesn't seem to be any real nice solution.

Even though xml2 is deprecated and may go away for 8.4, I think this is
important to fix in the back branches.  Failing to use the -f flags for
instance could be resulting in outright wrong code, and we'd be unlikely
to notice since there's no regression test at all for this module.

Thoughts?
		regards, tom lane