Correctly configure NXT_ENABLE_ASSERTS on multiconfiguration generators (#88)

And also set a default build type for single-configuration generators
diff --git a/src/common/Math.cpp b/src/common/Math.cpp
index da5455e..5fc2a40 100644
--- a/src/common/Math.cpp
+++ b/src/common/Math.cpp
@@ -62,7 +62,9 @@
 }
 
 uint32_t Align(uint32_t value, size_t alignment) {
+    ASSERT(alignment <= UINT32_MAX);
     ASSERT(IsPowerOfTwo(alignment));
     ASSERT(alignment != 0);
-    return (value + (alignment - 1)) & ~(alignment - 1);
+    uint32_t alignment32 = static_cast<uint32_t>(alignment);
+    return (value + (alignment32 - 1)) & ~(alignment32 - 1);
 }