build: warn about variable length arrays

configure.ac

@@ -210,6 +210,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wall],[CXXFLAGS="$CXXFLAGS -Wall"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wextra],[CXXFLAGS="$CXXFLAGS -Wextra"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wformat],[CXXFLAGS="$CXXFLAGS -Wformat"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wvla],[CXXFLAGS="$CXXFLAGS -Wvla"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wformat-security],[CXXFLAGS="$CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
 
   ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all