diff --git a/configure.ac b/configure.ac
index 39ac0a4e..9fb64e41 100644
--- a/configure.ac
+++ b/configure.ac
@@ -275,6 +275,7 @@ dnl
 dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 dnl ================================================================
 
+AX_CHECK_COMPILE_FLAG([-Wimplicit],              [CFLAGS="-Wimplicit $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wmissing-prototypes],    [CFLAGS="-Wmissing-prototypes $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wreturn-type],           [CFLAGS="-Wreturn-type $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wuninitialized],         [CFLAGS="-Wuninitialized $CFLAGS"])