switch to C99 breaks DFA tests

[Assaf Gordon]

Hello all,

I noticed that some standalone tests now fail because they use c99
constructs.

For example:

    ./gnulib-tool --create-testdir --dir test1 dfa
    cd test1
    ./configure
    make

fails with:
    dfa.c: In function ‘fillset’:
    dfa.c:727:3: error: ‘for’ loop initial declarations are only \
                                              allowed in C99 mode
       for (int i = 0; i < CHARCLASS_WORDS; i++)
       ^

as a work-around, running with
   make CFLAGS="-std=c99"
works fine.

regards,
 - assaf