CMakeLists: Fallback to RelWithDebInfo

CMakeLists: Fallback to RelWithDebInfo

Currently, if .git exists, we set CMAKE_BUILD_TYPE=Debug.
Otherwise, we leave it empty and no optimization flags will
be used.
With this change, the fallback CMAKE_BUILD_TYPE is set
to RelWithDebInfo instead.

From CMAKE_BUILD_TYPE manual:
The default value is often an empty string, but this is usually not
desirable and one of the other standard build types is usually more appropriate.

Note: One notable change is that -DNDEBUG will now be defined.

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