[Bug c/67093] incorrect -Wnonnull text for execl family of functions

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67093

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
execl is a GCC builtin and GCC issues the -Wformat warning regardless of its
declaration:

$ cat t.c && ~/bin/gcc-5.1.0/bin/gcc -Wall t.c
extern int execl (const char*, const char*, ...);

int main (void) {
    return execl ("/bin/echo", (char*)0);
}
t.c: In function ‘main’:
t.c:4:5: warning: not enough variable arguments to fit a sentinel [-Wformat=]
     return execl ("/bin/echo", (char*)0);
     ^

You're right though that the -Wnonnull warning is due to the nonnull attribute.