Add CMake flag PIP_OPTIONS to allow for passing specific options to pip during the install

[matthewfeickert]

In Issue #1641 @adriansev has explained that for software maintainers installing things at the system level there can be clear reasons why one would want to install the Python bindings with the --ignore-installed pip option (e.g. https://github.com/matthewfeickert/cvmfs-venv#disadvantages) instead of the --force-reinstall option

xrootd/bindings/python/CMakeLists.txt

Lines 89 to 98 in 3d64cda

	# Use --force-reinstall to ensure a clean install if a rebuild needs to happen
	install(
	CODE
	"EXECUTE_PROCESS(
	COMMAND ${PYTHON_EXECUTABLE} -m pip install \
	${PIP_INSTALL_VERBOSE_FLAG} \
	--force-reinstall \
	--prefix \$ENV{DESTDIR}/${CMAKE_INSTALL_PREFIX} \
	${CMAKE_CURRENT_BINARY_DIR}
	)"

Instead of trying to guess what users may or may not need, he also points out that by adding a PIP_OPTIONS CMake option this would allow expert users to exactly choose what they want, making it much more adaptable to installs across the board from a local user to a system admin.

This seems like a great idea to me, that should also be easy to implement. 👍

[adriansev]

#1647 :)