Disables non-POSIX warning when using gnu wildcard

commit #e68fd9db914b446867564d5c3695e1e6fac0927f
The following warning could previously be seen when building zoneminder:
```db/Makefile.am:13: wildcard zm_update-*.sql: non-POSIX variable name
db/Makefile.am:13: (probably a GNU make extension)```

This request removes the warning.