Use NDEBUG instead of _DEBUG macro to detect debug build

Closes #92 thanks @daemyung !
diff --git a/src/vk_mem_alloc.h b/src/vk_mem_alloc.h
index 32258b4..97f7e6d 100644
--- a/src/vk_mem_alloc.h
+++ b/src/vk_mem_alloc.h
@@ -3584,20 +3584,20 @@
 

 // Normal assert to check for programmer's errors, especially in Debug configuration.

 #ifndef VMA_ASSERT

-   #ifdef _DEBUG

-       #define VMA_ASSERT(expr)         assert(expr)

-   #else

+   #ifdef NDEBUG

        #define VMA_ASSERT(expr)

+   #else

+       #define VMA_ASSERT(expr)         assert(expr)

    #endif

 #endif

 

 // Assert that will be called very often, like inside data structures e.g. operator[].

 // Making it non-empty can make program slow.

 #ifndef VMA_HEAVY_ASSERT

-   #ifdef _DEBUG

-       #define VMA_HEAVY_ASSERT(expr)   //VMA_ASSERT(expr)

-   #else

+   #ifdef NDEBUG

        #define VMA_HEAVY_ASSERT(expr)

+   #else

+       #define VMA_HEAVY_ASSERT(expr)   //VMA_ASSERT(expr)

    #endif

 #endif