build_cffi_imports: fix determining CFLAGS

The previous logic implicitly assumed that CFLAGS was either unset or
started with a space, breaking the MacPorts build. In addition, it
also extracted the version info from the _running_ interpreter rather
than the _built_ one.

branch : py3.8-fix-cflags
2 jobs for branch/py3.8-fix-cflags in 29 minutes and 11 seconds (queued for 3 seconds)