configure: Add -Werror=bad-function-cast for --enable-warnings-as-error.

configure.ac

@@ -1010,8 +1010,9 @@ AC_ARG_ENABLE(warnings-as-error,
       AS_HELP_STRING([--enable-warnings-as-error],[Treat acceptable warnings as error (default in debug)]),, [enable_warnings_as_error="$enable_debug"])
 AS_IF([test "${enable_warnings_as_error}" = "yes"], [
     RDC_PROG_CC_WERRORFLAGS([unused-function unused-label unused-value unused-parameter unused-variable unused \
-     missing-braces nonnull parentheses uninitialized address enum-compare undef comment switch ignored])
-    ])
+     missing-braces nonnull parentheses uninitialized address enum-compare undef comment switch \
+     bad-function-cast ignored])
+])
 
 dnl Check for various optimization flags
 AC_CACHE_CHECK([if \$CC accepts -Os],