The problem was that Ant was looking at a Java 1.6 install on my computer (that I hadn't realized existed), and since my $JAVA_HOME variable was not set, it was unable to find the 1.7 install. I added a $JAVA_HOME variable by adding the following line to my .bash_profile file:
export JAVA_HOME='/Library/Java/JavaVirtualMachines/jdk1.7.0_40.jdk/Contents/Home'
This rectified the problem, and the project built successfully.
I realized that it actually informed me of the Java version at the very top of my Ant output originally, but I hadn't previously noticed it:
Detected Java version: 1.6 in: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
Some additional notes that may be helpful to users with similar issues:
-According to this, Ant will only utilize the value in the "executable" field of the javac section of the build-common.xml file if the "fork" field is set to yes. fork defaults to "no" if not listed, so if you don't add it and set it to "yes", the "executable" value will be ignored.
-On a Mac, your correct javac executable can likely be reached via /usr/bin/javac. On the command line, you can run
which javac
to find the path to the file being executed at the command line, and of course
javac -version
to find its version number. As noted above, Ant does not look at the javac in your command line PATH, but rather at JAVA_HOME.
Thanks to everyone for their help!