Can not build Bzr Emacs with libxml2 support on MacOSX

[CHENG Gao <chenggao@royau.me>]

Since I want to use shr as html renderer, I tried to build Bzr Emacs
with libxml2 support.

Some info:
MacOSX 10.7.2
libxml2 lib in /usr/lib (2.7.3)
libxml2 header files in /usr/include/libxml2

No matter if I add --with-xml2, it seems ./configure never checks xml2
(and other features like png, tiff, jpeg etc.). I took a look at
configure.in and found something as below:

,----
| dnl _ON results in a '--without' option in the --help output, so
| dnl the help text should refer to "don't compile", etc.
| OPTION_DEFAULT_ON([xpm],[don't compile with XPM image support])
| OPTION_DEFAULT_ON([jpeg],[don't compile with JPEG image support])
| OPTION_DEFAULT_ON([tiff],[don't compile with TIFF image support])
| OPTION_DEFAULT_ON([gif],[don't compile with GIF image support])
| OPTION_DEFAULT_ON([png],[don't compile with PNG image support])
| OPTION_DEFAULT_ON([rsvg],[don't compile with SVG image support])
| OPTION_DEFAULT_ON([xml2],[don't compile with XML parsing support])
| OPTION_DEFAULT_ON([imagemagick],[don't compile with ImageMagick image support])
`----

My understanding is they are default to off (--without options.). But
why does explicitly setting --with option not change these?

I don't know if it's problem of Macosx since I can build on Gentoo
without any problem.

Please kindly help me to build Emacs with libxml2 support.

TIA.

-- 
Volo, non valeo