Ant build under Eclipse - Error running javac.exe compiler

I ran into this problem twice, where the ant build script executes perfectly under the command prompt, but fails under Eclipse with the message - "Error running javac.exe compiler". It took me a while to figure out the problem, and it turns out that the problem arises from the javac task in ant. If the task contains an attribute fork="yes", then ant tries to spawn a new instance of javac to compile the source. Now if the referenced Java library in the project is a JRE, Ant can't find javac and therefore gives the above-mentioned error. If the parameter is changed to fork="no", then the build completes successfully, but a better solution is to include the JDK - and not the default JRE - as the referenced library for the project.