I know this is sort of a late reply, but other people might run into the same issue and find this thread while looking for a solution (just as I did):
Code:
yum install libXtst libXtst-devel libXext libXext-devel
cd /path/to/synergy/source
export CPPFLAGS='-I/usr/lib/X11'
./configure --prefix=/usr/local/synergy
sed -e 's/X_DISPLAY_MISSING 1/X_DISPLAY_MISSING 0/' -i'~' config.h
make
make install
You're welcome.