cmake: Fix ISA-L build on arm

cmake: Fix ISA-L build on arm

A typo in CFLAGS means we're passing an empty string to configure_cmd.
We are then overwriting the build environment CFLAGS with our empty string CFLAGS,
which can result in build failures in certain environments, as seen in the tracker.
This fix gets any build environment CFLAGS and appends the other flags
we want to use when building ISA-L 2.32.0

Fixes: https://tracker.ceph.com/issues/76389
Signed-off-by: Jamie Pryde <jamiepry@uk.ibm.com>