Source code tarball installation under root user:

• Unpack archive with 'tar zxvf filename' or 'gunzip -c filename | tar xvf -'
• Go to xnc-5.x.x directory
• Run './configure'. For configure options see output of './configure --help'
• Run 'make'
• Run 'make install' under root. This operation install files to Your computer.
• You need to run 'xncsetup' for each user who wants to run xnc.
• Configure Your environment with xncsetup and press 'Save' button.

PS: If you have troubles compiling XNC try run
'./configure --disable-shared' and then make again.

Source code tarball installation for local user without root access:

If you want to install xnc to another directory that default one (/usr), then read this part of instructions:
Assume that you want to install xnc locally to your home directory and don't have 'root' access.
For example your home directory is '/home/leo'.

• Unpack archive with 'tar zxvf filename' or 'gunzip -c filename | tar xvf -'
• Go to xnc-5.x.x directory 
• Run './configure --prefix=/home/leo'. For more configure options see output of './configure --help'
• Run 'make' 
• Run 'make install'. This operation install files to Your computer.
• You need to run 'xncsetup' before running xnc. Do it under X window system because xncsetup requires connection to X server. XNC installs itself in the following directories:
• /home/leo/bin - binaries
• /home/leo/lib/xnc - libraries
• /home/leo/share/man/man1 - man pages
• /home/leo/share/doc/xnc-5.x.x - documentation
• /home/leo/share/locale - localization
• Configure Your environment with xncsetup and press 'Save' button.
• Make sure that you have '/home/leo/bin' directory in your $PATH environment.
• Or add with 'export PATH=/home/leo/bin:$PATH' into your local startup script.