No x86 support for SketchUp 2017 C API?

That is a known “problem”. While the SketchUp 2017 GUI application dropped support for old operating systems and uncommon hardware (x86 32bit, old OpenGL versions), also the SDK hasn’t been released for x86 32bit.
That’s unfortunate since the SDK has different scope than the GUI app, and affects all third-party applications that are not yet 64bit.