Warn if VLAs are used

We don't intend to use C99 variable-length arrays, so make the
compiler complain about them.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 19801e8..157a903 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,6 +128,9 @@
     execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion
                     OUTPUT_VARIABLE GCC_VERSION)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wwrite-strings")
+    if (GCC_VERSION VERSION_GREATER 4.3 OR GCC_VERSION VERSION_EQUAL 4.3)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wvla")
+    endif()
     if (GCC_VERSION VERSION_GREATER 4.5 OR GCC_VERSION VERSION_EQUAL 4.5)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wlogical-op")
     endif()
@@ -144,7 +147,7 @@
 endif(CMAKE_COMPILER_IS_GNU)
 
 if(CMAKE_COMPILER_IS_CLANG)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wwrite-strings -Wpointer-arith -Wimplicit-fallthrough -Wshadow")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wwrite-strings -Wpointer-arith -Wimplicit-fallthrough -Wshadow -Wvla")
     set(CMAKE_C_FLAGS_RELEASE     "-O2")
     set(CMAKE_C_FLAGS_DEBUG       "-O0 -g3")
     set(CMAKE_C_FLAGS_COVERAGE    "-O0 -g3 --coverage")