Default CFLAGS to -std=c99 instead of -ansi.

Default CFLAGS to -std=c99 instead of -ansi.

gcc -ansi -pedantic issued a few warnings about C90 not supporting the
"ll" printf length modifier.  Ralph pointed out that
http://www.unix.org/version4/overview.html says:

    The following source code portability standards lie at the core of
    the Single UNIX Specification:

        POSIX.1-2008

    (This is technically identical to the Base Specifications, Issue 7;
    they are one and the same document.)

        The ISO/IEC 9899:1999 standard

So perhaps we're wrong to expect modern POSIX code to compile as C90.