If no JAVA_HOME or ENV{JAVA_HOME} is set then try running this
tool to get the system default Java home.
Suggested-by: Larry Shaffer <larrys@dakotacarto.com>
Add a helper module CMakeFindJavaCommon shared between FindJava
and FindJNI to select a JAVA_HOME value. Prefer a CMake variable
or cache entry first, then an environment variable.