Fix #69: use shiboken target over variable

[no-preserve-root]

Fixes #69 and should be downward compatible.

[dirk-thomas]

Thanks for the patch.

[dirk-thomas]

Cherry-picked to the crystal-devel branch: fa91148.

[stertingen]

Although the variable SHIBOKEN_BINARY is now always correctly set, there are a few variables that are used but potentially empty:

• SHIBOKEN_LIBRARY (lines 19, 20, 121, prevents output in line 20)
• SHIKOBEN_PYTHON_LIBRARIES (line 120)
• SHIBOKEN_INCLUDE_DIR (line 96)

[no-preserve-root]

@stertingen you're right. I think that splitting the CMake file for shiboken2 >= 5.12.2 and one for the legacy version is the way to go; essentially, the CMake integration is fundamentally different and branching on the version is substantially easier than trying to emulate the legacy variables.

I might get a version to work over the next days and will submit a pull request then.

[dirk-thomas]

there are a few variables that are used but potentially empty

See #78 for a new ticket about those.