erzapito,
It looks like the build system might not be creating this file at the moment even though it is needed for compilation. As a work-around, create a file svnversion.h in your root trunk directory with a single line as the content:
#define SVN "4557+"
You can safely ignore any warnings generated during compile.
PS: feel free to drop by on our IRC chat channel: #ufo:ai on server freenode if you run into more issues, there is quite often someone there who can help (or failing that stick to these forums).