This fixes a bug of not noticing the necessity to have -lm
    when compiling sacprelude.  The reason for this is that

    $ cat a.c
      #include <math.h>
        int main(void) {
        return log2(1.4);
      }
    $ gcc -Wall -Wextra -Werror -std=c99 a.c
    $ echo $?
    $ 0
    $ # success
    $ # whereas if we compile the same file like this:
    $ gcc -Wall -Wextra -Werror -std=c99 -fno-builtin -fno-builtin-function a.c
    $
    $/tmp/ccaHuGXn.o: In function `main':
    $a.c:(.text+0x1c): undefined reference to `log2'
    $collect2: error: ld returned 1 exit status
    $ echo $?
    $ 1
    $ # fail

    Therefore we want to include -fno-builtin whenever it is supported
    by the compiler.