I realized that some unix system distribution does not provide xmlwriter.h in standard libxml2-devel 
package.

For instance, scientific linux (also PSI linux) does not.
I installed it from source.

Macintosh has xmlreader.h by default, and does not xmlwriter.h.
I installed it by using FINK.
(FINK is a unix package porter like CYGWIN.)

Someone may ask about it someday. 
A problem is that compiler will find xmlwriter.h even if the system does not have it, because ROME has.
This may make it hard that users find out the problem.


How to install libxml2.
----  UNIX OS ----
In most cases, to use standard way of the system is good. But some system does not provide xmlwriter.h. In this case you may need to 
install it from source.
1. download source from http://xmlsoft.org/.
2. decompress it and go to created directory.
3. % ./configure [options as you like];
4. % make;
5. become super user, if you are going to install it in system.
6. % make install

---- MacOSX ----
By default, Macintosh does not have xmlwriter.h. You can follow the same way as other UNIX OS. And other way is to use FINK. (I hope 
many users already installed it.)
1. install FINK (http://fink.sourceforge.net)
2. become super user.
3. apt-get update;
4. apt-get install libxml2 libxml2-shlibs libxml2-bin
If you are goint to use also MySQL.
5. apt-get install mysql12-dev mysql12-shlibs
(* package name can be changed in future, you may find it with "apt-cache search libxml2" or "apt-cache search mysql")