Hmmm ... bash in usually /bin/bash, not /usr/local/bin/bash. Me thinks there's something a bit funny with that build script, or it was written for a different platform. Also, C source files are generally not referred to as "scripts", since that would cause confusion with C shell scripts as well as being incorrect - C is a compiled language, which is what the configure file is trying to set up for you (probably so you can run "make" next, which actually does the compilation).
Anyhow, have you tried using the nginx-webdav package from the AUR
? I run Arch myself, but I haven't built too many packages from the AUR, so I can't comment much on the quality. There's a section in the Arch wiki describing how to build AUR packages.
BTW, is there some particular reason you need Nginx? Apache has had pretty mature WebDAV support for quite a long time, and it might be a bit easier to get going. I know that Nginx is the new shiny, but I'm curious if there's a requirement for it in your scenario (honest question - I'm not saying at all that it's the wrong choice for you).