-DASSERTF_DISABLE and assert_*() causes implicit declaration of function about assertf.h

[leiless]

Bug fixed, tested in macOS

# lei @ lei-mbp in /tmp/t [22:16:17]
$ gcc -Wall -DASSERTF_DISABLE foo.c

# lei @ lei-mbp in /tmp/t [22:16:19]
$ ls -l a.out
-rwxr-xr-x  1 lei  wheel  16576 May 19 22:16 a.out

# lei @ lei-mbp in /tmp/t [22:16:24]
$ nm a.out
0000000100003f70 T ___vunused
0000000100000000 T __mh_execute_header
0000000100003f90 T _main
                 U dyld_stub_binder

# lei @ lei-mbp in /tmp/t [22:16:25]
$ strings a.out
lhs: %s%d%s rhs: %s%d%s

# lei @ lei-mbp in /tmp/t [22:16:26]
$ gcc -Wall -Wextra -O2 -DASSERTF_DISABLE foo.c

# lei @ lei-mbp in /tmp/t [22:16:41]
$ ls -l a.out
-rwxr-xr-x  1 lei  wheel  16576 May 19 22:16 a.out

# lei @ lei-mbp in /tmp/t [22:16:44]
$ nm a.out
0000000100003fa0 T ___vunused
0000000100000000 T __mh_execute_header
0000000100003fb0 T _main
                 U dyld_stub_binder

# lei @ lei-mbp in /tmp/t [22:16:44]
$ strings a.out