Fix a build problem on Mac OS X.

library/int.m:
    LLONG_{MIN,MAX} are the standard (C99) names for the limits
    of the long long int type; use them in preference to LONG_LONG_{MIN,MAX},
    which are a GNU C thing.

    Make the use of the above macros conditional on them actually being
    defined since we cannot guarantee that they will be defined if the
    C compiler is not in C99 mode (or something similar to it).