define WEBP_RESTRICT for MSVC

__restrict is supported:
https://docs.microsoft.com/en-us/cpp/cpp/extension-restrict?view=msvc-160

+ add a comment and simplify the __restrict__ check, clang defines
__GNUC__

Change-Id: I76d8d3cbd1b730f492c6fc0b2114f1897886b677

src/utils/utils.h

@@ -25,20 +25,27 @@
 #include "src/dsp/dsp.h"
 #include "src/webp/types.h"
 
-//------------------------------------------------------------------------------
+#ifdef __cplusplus
-// restrict
+extern "C" {
+#endif
 
-#if defined(__GNUC__) || defined(__clang__)
+//------------------------------------------------------------------------------
+// WEBP_RESTRICT
+
+// Declares a pointer with the restrict type qualifier if available.
+// This allows code to hint to the compiler that only this pointer references a
+// particular object or memory region within the scope of the block in which it
+// is declared. This may allow for improved optimizations due to the lack of
+// pointer aliasing. See also:
+// https://en.cppreference.com/w/c/language/restrict
+#if defined(__GNUC__)
+#define WEBP_RESTRICT __restrict__
+#elif defined(_MSC_VER)
 #define WEBP_RESTRICT __restrict
 #else
 #define WEBP_RESTRICT
 #endif
 
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 //------------------------------------------------------------------------------
 // Memory allocation