Fix m4 build with the newest gnulib

With the newest gnulib, a build of m4 from git fails: make[2]: Entering directory '/tmp/m4/src' CC m4.o CC builtin.o builtin.c: In function 'm4_syscmd': builtin.c:968:44: warning: passing argument 3 of 'execute' from incompatible pointer type [-Wincompatible-pointer-types] stat

Re: Fix m4 build with the newest gnulib

Thanks, I installed that in the 1.4 branch.