cpupower: Improve Python binding's Makefile

Add a few build variables to make it easier for distributions to
package the bindings. Allow current variables to be overwritten by
environment variables that are passed to make.

CCing Thorsten Leemhuis <linux@leemhuis.info>.

https://lore.kernel.org/r/20250624204105.457971-1-jwyatt@redhat.com


Signed-off-by: John B. Wyatt IV <jwyatt@redhat.com>
Signed-off-by: John B. Wyatt IV <sageofredondo@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>