diff --git a/configure.ac b/configure.ac
index 79d06985..7588ff8f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -90,6 +90,7 @@ fi
 dnl
 dnl Check for programs
 dnl
+CFLAGS="$CFLAGS -Werror=implicit-function-declaration"
 AC_PROG_CC
 AM_CFLAGS=""
 AC_HEADER_STDC