missing directory libxml in source tree, failing to compile

slackware 12.0.
I think I have all the dependencies. I compiled with binutils 2.17, 2.15, and 2.18. Using 2.18 now, here is my insuperable error:

scons: `libs/libsndfile/libsndfile-ardour.so’ is up to date.
g++ -o libs/pbd/command.os -c -O3 -fomit-frame-pointer -ffast-math -fstrength-reduce -pipe -DARCH_X86 -mmmx -m3dnow -march=i686 -msse -mfpmath=sse -DUSE_XMMINTRIN -DBUILD_SSE_OPTIMIZATIONS -Wall -DHAVE_LIBLO -DENABLE_NLS -DHAVE_GETMNTENT -Woverloaded-virtual -DPACKAGE=“libpbd” -D_REENTRANT -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -DLIBSIGC_DISABLE_DEPRECATED -DHAVE_EXECINFO -fPIC -I/usr/include/glib-2.0 -Ilibs/sigc++2 -Ilibs/glibmm2 -I/usr/lib/glib-2.0/include -Ilibs/pbd -I/usr/include/libxml2 libs/pbd/command.cc
In file included from libs/pbd/command.cc:21:

libs/pbd/pbd/xml++.h:13:27: error: libxml/parser.h: ENOENT
libs/pbd/pbd/xml++.h:14:25: error: libxml/tree.h: ENOENT
scons: *** [libs/pbd/command.os] Error 1
scons: building terminated because of errors.
The last section is what I think is the problem.
Indeed, I can’t find libxml/ in libs/pbd/pbd/ directory of the source tree. It only goes as deep as libs/pbd/pbd. Should libxml/ already be there? If I look at the file libs/pbd/pbd/xml++.h, it says to include libxml/tree.h and libxml/parser.h near the beginning of the file.

I used the source package both from the slacky.eu repository and from the ardour website, both have exact same problem.

You have probably not fulfilled all of the requirements mentioned on
the build page. Sounds as if you have not installed libxml-dev (aka libxml-devel).