All your ant stuff will work fine except the javac task which needs the tools.jar, located in the /lib directory from the JDK, JRE is not sufficient in that case. Therefore the hint from ant : “Unable to find a javac compiler;…”
When working with Eclipse the default setting points to your JRE installation.
So, one of your first steps after starting Eclipse for the first time should be :
Window > Preferences > Java > Installed JREs and change the settings from JRE to JDK.
Alternatively use :
Window > Preferences > Ant > Runtime > Classpath > Global Entries
and add the tools.jar from your JDK/lib folder