cleanup: Use _Static_assert in gcc/clang.

Hopefully they don't throw warnings at us.

.github/scripts/cmake-osx

@@ -20,6 +20,11 @@ add_ld_flag -undefined error
 # Make compilation error on a warning
 add_flag -Werror
 
+# Allow _Static_assert. Supported C11 extensions are fine, since we have several
+# C99-only compilers we test against anyway. Anything that passes all the
+# compilers we use is fine.
+add_c_flag -Wno-c11-extensions
+
 cmake -B_build -H. \
   -DCMAKE_C_FLAGS="$C_FLAGS" \
   -DCMAKE_CXX_FLAGS="$CXX_FLAGS" \

toxcore/ccompat.h

@@ -67,11 +67,12 @@
 #if !defined(__cplusplus) || __cplusplus < 201103L
 #define nullptr NULL
 #ifndef static_assert
-#define STATIC_ASSERT_(cond, msg, line) typedef int static_assert_##line[(cond) ? 1 : -1]
-#define STATIC_ASSERT(cond, msg, line) STATIC_ASSERT_(cond, msg, line)
 #ifdef __GNUC__
-#define static_assert(cond, msg) __attribute__((__unused__)) STATIC_ASSERT(cond, msg, __LINE__)
+// We'll just assume gcc and clang support C11 _Static_assert.
+#define static_assert _Static_assert
 #else // !__GNUC__
+#define STATIC_ASSERT_(cond, msg, line) typedef int static_assert_##line[(cond) ? 1 : -1]
+#define STATIC_ASSERT(cond, msg, line) STATIC_ASSERT_(cond, msg, line)
 #define static_assert(cond, msg) STATIC_ASSERT(cond, msg, __LINE__)
 #endif // !__GNUC__
 #endif // !static_assert