JSON-based build tool?
Nokia/Qt just announced their "QBS" build system, which is JSON-based, and is actually a build engine intended for easy adoption by IDEs (a principal reason for adopting JSON).
Rather than a "pre-make" generator of Makefiles/vcprojs/etc. (like CMake and QMake, etc.), qbs is intended to actually be the build engine.
I think what you're really looking for is a build tool that doesn't force you to configure everything in XML?
In that case your best bet is Gradle. It doesn't have the same sort of adoption as ANT or Maven, but does have the advantage of learning lessons from both.
I would advise you to beware of straying from standard build tooling... It makes life tougher for others attempting to build your code. This is something Gradle solved by providing a build wrapper.