I think it would help if you just did a fresh attempt to patch and make and posted the output. I'm finding this hard to read because I see "can't find file to patch" followed by a make.
You need to be in the source code directory that has a subdirectory trunk containing common/xymonlaunch.c (or edit that path to reflect what you actually have checked out )
Then when attempting to make you need to have libxymon.h in your library path
I'm thinking that the last time I built from source I didn't have "trunk" in my path because I only downloaded the *contents* of the trunk directory, so you might have to tweak the patch to reflect this.
(If you're not familiar with SVN the repository layout may be confusing. Take a look at http://svnbook.red-bean.com/en/1.7/svn.branchmerge.maint.html#svn.branchmerg... )