configure's dependency on pkg-config

[Stephen Berman]

I built a GNU/Linux system from sources (Linux From Scratch) and then
built the current Emacs pretest on it with configure && make, but was
surprised that configure failed to use libxml2, although I had built and
installed the libxml2 files in the standard locations.  I then
reconfigured Emacs, passing the libxml2 requirements to configure via
CPPFLAGS, LDFLAGS and LIBS, but it still failed to use libxml2.  Then I
looked at configure and found the reason:

  if test "$PKG_CONFIG" = "no" ; then
     HAVE_LIBXML2=no

I don't have pkg-config installed on this system.  I tried passing
PKG_CONFIG=1 to configure, but that didn't help, because if that
variable is set, configure looks for the pkg-config executable.
Finally, I simply hacked out the pkg-config bits around the libxml2
tests in configure and passed the requirements via LIBXML2_CFLAGS and
LIBXML2_LIBS, and this worked.  

Is there a better way to override configure's dependency on pkg-config
for libraries such as libxml2, so that it would suffice to pass the
standard variable CPPFLAGS, LDFLAGS and LIBS?  Or if there isn't, should
there be?

Steve Berman