Add NXT_UNUSED to silence unused warnings

src/common/Assert.h

@@ -55,7 +55,7 @@
 #    else
 #        define NXT_ASSERT_CALLSITE_HELPER(file, func, line, condition) \
             do {                                                        \
-                (void)sizeof(condition);                                \
+                NXT_UNUSED(sizeof(condition));                          \
             } while (NXT_ASSERT_LOOP_CONDITION)
 #    endif
 #endif

src/common/Compiler.h

@@ -22,6 +22,7 @@
 //  - NXT_NO_DISCARD: An attribute that is C++17 [[nodiscard]] where available
 //  - NXT_(UN)?LIKELY(EXPR): Where available, hints the compiler that the expression will be true
 //      (resp. false) to help it generate code that leads to better branch prediction.
+//  - NXT_UNUSED(EXPR): Prevents unused variable/expression warnings on EXPR.
 
 // Clang and GCC
 #if defined(__GNUC__)
@@ -72,6 +73,9 @@ extern void __cdecl __debugbreak(void);
 #    error "Unsupported compiler"
 #endif
 
+// It seems that (void) EXPR works on all compilers to silence the unused variable warning.
+#define NXT_UNUSED(EXPR) (void)EXPR
+
 // Add noop replacements for macros for features that aren't supported by the compiler.
 #if !defined(NXT_LIKELY)
 #    define NXT_LIKELY(X) X