CMakeLists.txt: increase verbosity for selected allocator

CMakeLists.txt: increase verbosity for selected allocator

Unless the allocator was set on command line, we will select one based on the following order:
```
"specify memory allocator to use. currently tcmalloc, tcmalloc_minimal, \
jemalloc, and libc is supported. if not specified, will try to find tcmalloc, \
and then jemalloc. If neither of then is found. use the one in libc.")
```
with this change, cmake will explicitly message the compiler selected,
otherwise we have no option to identify the one which is being used.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>