Be more library friendly, by not aborting in library errors.
spice_common now includes a proper log handler that will abort by
default when reaching a warning.
SPICE_ABORT_LEVEL can be changed to modify run-time abort level.
SPICE_DEBUG_LEVEL can be changed to be more verbose. By default, only
log level more importants than WARNING.
Only memory-related functions are allowed to abort(), since they are
not recoverable errors in the library.
Since some spice C++ code is using code from common/, the C
functions need to be marked as such for the C++ compiler, otherwise
we'll get linkage issues.
