configure: Fix the way how cc.mk variables are read

The 7a186735 didn't consider a fact that vars like $CC can also hold
an actual list of arguments to the compiler (instead of having them as
part of $CFLAGS for instance). So make sure something like:

CC=gcc -Wwhatever -ffoo ...

is slurped by source properly.

Change-Id: I7d9a26d318bef42655a3dde127db2226c9759949
Signed-off-by: Michal Berger <michal.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/22030


Community-CI: Mellanox Build Bot
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>