configure: add -Werror-unknown-warning-option

this is needed so the other warning flags we test raise an error and
we can detect whether they're supported.

src/configure

@@ -3301,7 +3301,7 @@ if test "${enable_savetocurrent+set}" = set; then :
 fi
 
 
-DESIRED_FLAGS="-Wno-unused-but-set-variable"
+DESIRED_FLAGS="-Werror-unknown-warning-option -Wno-unused-but-set-variable"
 for flag in $DESIRED_FLAGS; do
 
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking to see if compiler understands $flag" >&5

src/configure.ac

@@ -16,7 +16,7 @@ AC_HELP_STRING([--enable-savetocurrent],
   [saves the current session file to the directory reaver was started from])
 , [ CFLAGS="$CFLAGS -DSAVETOCURRENT" ])
 
-DESIRED_FLAGS="-Wno-unused-but-set-variable"
+DESIRED_FLAGS="-Werror-unknown-warning-option -Wno-unused-but-set-variable"
 for flag in $DESIRED_FLAGS; do
   AS_COMPILER_FLAG([$flag], [CFLAGS="$CFLAGS $flag"])
 done