Force an error when a function prototype is missing

If someone understands the stochastic process whereby GCC decides to
apply this function or not, please let me know.

configure.ac

@@ -933,7 +933,7 @@ if test "${ac_cv_c_ntohl_sys_param_h}" != "no"; then
 fi
 CFLAGS="${CFLAGS_save}"
 
-RDC_PROG_CC_WFLAGS([all extra sign-compare undef pointer-arith bad-function-cast cast-align write-strings missing-prototypes volatile-register-var])
+RDC_PROG_CC_WFLAGS([all extra sign-compare undef pointer-arith bad-function-cast cast-align write-strings missing-prototypes volatile-register-var error-implicit-function-declaration])
 RDC_PROG_CC_FLAGS_IFELSE([-pipe])
 
 dnl Check for various optimization flags