[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.

ntoskrnl/CMakeLists.txt

@@ -1,6 +1,11 @@
 
 PROJECT(NTOS)
 
+if (GCC)
+    # Enable this again. CORE-17637
+    add_compile_options(-Wunused-result)
+endif()
+
 include(ntos.cmake)
 
 if (CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")

sdk/cmake/gcc.cmake

@@ -102,6 +102,7 @@ add_compile_options(-Wno-char-subscripts -Wno-multichar -Wno-unused-value)
 add_compile_options(-Wno-unused-const-variable)
 add_compile_options(-Wno-unused-local-typedefs)
 add_compile_options(-Wno-deprecated)
+add_compile_options(-Wno-unused-result) # FIXME To be removed when CORE-17637 is resolved
 
 if(NOT CMAKE_C_COMPILER_ID STREQUAL "Clang")
     add_compile_options(-Wno-maybe-uninitialized)

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

sdk/lib/rtl/CMakeLists.txt

@@ -5,6 +5,11 @@ add_definitions(
     -D_NTSYSTEM_
     -D_NTDLLBUILD_)
 
+if (GCC)
+    # Enable this again. CORE-17637
+    add_compile_options(-Wunused-result)
+endif()
+
 list(APPEND SOURCE
     access.c
     acl.c