MINOR Replacing cmake ant plugin with cmake executable

The CMake ant plugin is not working together with CMake 2.8.10
or higher. Therefore the change to the exec task together
with the CMake exectuable located in the Path was necessary.

Change-Id: Ic8bbb90615f1aa6fbb047e8c59a824b4e37e52e0

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1518247 13f79535-47bb-0310-9956-ffa450edef68