Makefile: Generate libiio.pc from a template

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Makefile: Add install-sysroot and uninstall-sysroot targets

Those targets can be used to install the files used by the applications
that want to use libiio.

This does not change the behaviour of the "install" target.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>