Avoid a zero-length array for the FlexArray definition

Newer GCCs are warning about FlexArray being used at the end of another struct.
Though not the most ideal to always allocate space for at least 1 element,
actual 0-sized uses are rare and for smaller element types.

alc/alc.cpp

@@ -947,13 +947,9 @@ constexpr int alcEFXMinorVersion{0};
 
 
 /* To avoid extraneous allocations, a 0-sized FlexArray<ALCcontext*> is defined
- * globally as a sharable object. MSVC warns that a zero-sized array will have
- * zero objects here, so silence that.
+ * globally as a sharable object.
  */
-DIAGNOSTIC_PUSH
-msc_pragma(warning(disable : 4815))
 al::FlexArray<ALCcontext*> EmptyContextArray{0u};
-DIAGNOSTIC_POP
 
 
 using DeviceRef = al::intrusive_ptr<ALCdevice>;

common/almalloc.h

@@ -179,11 +179,10 @@ struct FlexArray {
 
 
     const index_type mSize;
-DIAGNOSTIC_PUSH
-std_pragma("GCC diagnostic ignored \"-Wpedantic\"")
-msc_pragma(warning(disable : 4200))
-    alignas(alignment) element_type mArray[0];
-DIAGNOSTIC_POP
+    union {
+        char mDummy;
+        alignas(alignment) element_type mArray[1];
+    };
 
     static std::unique_ptr<FlexArray> Create(index_type count)
     {