Win32: Build against libxml2, iconv, and zlib

DocFormats now builds successfully on Windows using Visual Studio 2013,
provided you have the following directory hierarchy set up with the
appropriate files under the root of the repository:

libs/iconv
libs/libxml2
libs/zlib

Pre-built versions of these libraries can be obtained from
http://www.zlatkovic.com/libxml.en.html; the files you want are:

iconv-1.9.2.win32.zip
libxml2-2.7.8.win32.zip
zlib-1.2.5.win32.zip

Unzip each of these files, rename the directories to exclude the version
number, and place then in 'libs' as above. So for example you would have
C:\dev\Socrates\libs\iconv\include\iconv.h and similar files.

The location of the libs directory can be changed by editing the
top-level CMakeLists.txt file.

As part of this commit, a number of other CMakeLists.txt files have been
updated so they use the INCLUDE_DIRS and LIB_DIRS variables whenever
they refer to the above libraries.