[SDK] Add GCC overrides for SAL annotations

GCC has some functions, variables & type attributes which can be used as aliases
for some of the SAL annotations. Although it's not as rich & precise, it's still useful
since we actually enable -Werror on GCC builds whereas we don't use such an option
on MSVC builds.

For now, _Must_inspect_result_ is aliased to warn_result_unused attribute.

sdk/include/psdk/gcc_sal.h

@@ -0,0 +1,22 @@
+/*
+ * PROJECT:     ReactOS PSDK
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Standard Annotation Language (SAL) definitions - GCC overrides
+ * COPYRIGHT:   2021 - Jérôme Gardou
+ */
+
+#pragma once
+
+#ifndef __GNUC__
+#error "Not for your compiler!"
+#endif
+
+#ifndef __has_attribute
+#pragma GCC warning "GCC without __has_attribute, no SAL niceties for you"
+#define __has_attribute(__x) 0
+#endif
+
+#if __has_attribute(warn_unused_result)
+# undef _Must_inspect_result_
+# define _Must_inspect_result_ __attribute__((__warn_unused_result__))
+#endif