java: require CMake 3.12

Ben Boeckelrequested to merge
ben.boeckel/vtk:java-requires-cmake-3.12 into master

CMake 3.12 introduced support for the SHELL: escaping syntax for target_compile_options. This is required to get CMake to accept the -source and -target flags correctly.

Backport: release

Edited by Ben Boeckel

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