libcamera: base: Remove custom __nodiscard attribute

__nodiscard was introduced for compatibility with C++14.
In C++17, there is an official attribute: [[nodiscard]].
Moreover, some libc implementations (like bionic) already define the
__nodiscard macro [1].

Since:
- libcamera builds with cpp_std=c++17
- [[nodiscard]] is already used in the android HAL (exif)

We should replace all usage __nodiscard of by [[nodiscard]] for
consistency.

Do the replacement and remove the no longer used compiler.h.

[1] https://android-review.googlesource.com/c/platform/bionic/+/3254860

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/base/compiler.h

@@ -1,14 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-/*
- * Copyright (C) 2021, Google Inc.
- *
- * Compiler support
- */
-
-#pragma once
-
-#if __cplusplus >= 201703L
-#define __nodiscard		[[nodiscard]]
-#else
-#define __nodiscard
-#endif

include/libcamera/base/meson.build

@@ -5,7 +5,6 @@ libcamera_base_include_dir = libcamera_include_dir / 'base'
 libcamera_base_public_headers = files([
     'bound_method.h',
     'class.h',
-    'compiler.h',
     'flags.h',
     'object.h',
     'shared_fd.h',

include/libcamera/base/unique_fd.h

@@ -10,7 +10,6 @@
 #include <utility>
 
 #include <libcamera/base/class.h>
-#include <libcamera/base/compiler.h>
 
 namespace libcamera {
 
@@ -43,7 +42,7 @@ public:
 		return *this;
 	}
 
-	__nodiscard int release()
+	[[nodiscard]] int release()
 	{
 		int fd = fd_;
 		fd_ = -1;