Otherwise, this header is installed if BUILD_PYTHON_INTERFACE, and not
if INSTALL_PYTHON_INTERFACE_ONLY.

But if we want to be able to have binary packages for:
- the main curve package
- its bindings for python 2
- its bindings for python 3

this header will be installed in the 2 last cases, which will conflict.

In short: if we set INSTALL_PYTHON_INTERFACE_ONLY, there should be only
stuff installed in PYTHON_SITELIB.