Install the source RPM for nginx and move it to you RPM build environment.
cd SOURCES/
tar xvzf nginx-1.6.3.tar.gz
cd nginx-1.6.3
git clone https://github.com/stnoonan/spnego-http-auth-nginx-module.git
cd ..
mv nginx-1.6.3 nginx-spnego-1.6.3
tar cvzf nginx-spnego-1.6.3.tar.gz nginx-spnego-1.6.3/