The first thing to do is find out where libxine.pc is installed. To do this you need the find command, so log into your terminal as root then try:
find / -name libxine.pc -print
This is asking your system to search through your entire directory tree for the missing file. Let's say you find the file in /usr/local/bin . If so you would configure Xine like this:
./configure PKG_CONFIG_PATH="/usr/local/bin"
You're passing the path of the file to ./configure, and notice the double quotation marks (""). Once you've got the path correct Xine should compile with few problems.
Once Xine is installed it's worth running xine-check to get some idea of how things went. Xine is a very stable program which I admire, but there are some minor things which will cause it to fail! It's a very good idea to read the documentation
here.