S4-piper cannot be built on RHEL 5 Linux

After cleaning all possible conflicting directories ~/.m2 and ~/.gradle.
./gradlew install fails on s4-edsl by throwing a NullPointer() exception.
The debug code for Gradle shows:

• Where:
  Build file '/home/user/piper/subprojects/s4-edsl/s4-edsl.gradle' line: 45

• What went wrong:
  Execution failed for task ':s4-edsl:generateSources'.
  > java.lang.NullPointerException (no error message)

And line 45 is:
mojo.execute();

This applies only to this linux box, since I was able to successfully build piper on a Mac with no problems.