our gnu C has not this option!

Index: configure
--- configure.orig
+++ configure
@@ -19658,7 +19658,7 @@ fi
 
 case "${ax_cv_c_compiler_vendor}" in
 gnu)
-    COMP_VENDOR_CFLAGS="-std=gnu99 -ffast-math -Wall -Wunused-variable -Wunused-but-set-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes $COMP_VENDOR_CFLAGS"
+    COMP_VENDOR_CFLAGS="-std=gnu99 -ffast-math -Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes $COMP_VENDOR_CFLAGS"
     if test "$enable_avx" = "yes" ; then
         COMP_VENDOR_CFLAGS="-mavx $COMP_VENDOR_CFLAGS"
     fi
