diff --git a/configure.ac b/configure.ac
index 42e38b2..efd1e79 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1046,7 +1046,8 @@ CHARYBDIS_C_GCC_TRY_FLAGS([-Wpointer-arith], charybdis_cv_c_gcc_w_pointer_arith)
 CHARYBDIS_C_GCC_TRY_FLAGS([-Wimplicit -Wnested-externs], charybdis_cv_c_gcc_w_implicit)
 CHARYBDIS_C_GCC_TRY_FLAGS([-Wcast-align], charybdis_cv_c_gcc_w_cast_align)
 CHARYBDIS_C_GCC_TRY_FLAGS([-Wcast-qual], charybdis_cv_c_gcc_w_cast_qual)
-CHARYBDIS_C_GCC_TRY_FLAGS([-Wwrite-strings], charybdis_cv_c_gcc_w_float_equal)
+CHARYBDIS_C_GCC_TRY_FLAGS([-Wwrite-strings], charybdis_cv_c_gcc_w_write_strings)
+CHARYBDIS_C_GCC_TRY_FLAGS([-Werror-implicit-function-declaration], charybdis_cv_c_gcc_w_error_implicit_function_declaration)
 CHARYBDIS_C_GCC_TRY_FLAGS([-Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations], charybdis_cv_c_gcc_prototypes)
 CHARYBDIS_C_GCC_TRY_FLAGS([-Wparenthesis], charybdis_cv_c_gcc_parenthesis)
 CHARYBDIS_C_GCC_TRY_FLAGS([-W -Wno-unused], charybdis_cv_c_gcc_w)