Support defining C99 bool as the BOOL type

Whether to use C99 bool can be set at build time using
-DOBJC_BOOL_IS_BOOL_MODE=auto/always/never.

Default is to use C99 bool if __OBJC_BOOL_IS_BOOL is defined and
set to 1, which is currently the case for Clang on newer Apple
platforms. This matches the behavior of Apple's libobjc (especially
when combined with STRICT_APPLE_COMPATIBILITY).

(See
https://www.jviotti.com/2024/01/05/is-objective-c-bool-a-boolean-type-it-depends.html
for more information about the __OBJC_BOOL_IS_BOOL definition.)

CMakeLists.txt

@@ -141,15 +141,23 @@ option(DEBUG_ARC_COMPAT
 option(ENABLE_OBJCXX "Enable support for Objective-C++" ON)
 option(TESTS "Enable building the tests")
 option(EMBEDDED_BLOCKS_RUNTIME "Include an embedded blocks runtime, rather than relying on libBlocksRuntime to supply it" ON)
-option(STRICT_APPLE_COMPATIBILITY "Use strict Apple compatibility, always defining BOOL as signed char" OFF)
+option(STRICT_APPLE_COMPATIBILITY "Use strict Apple compatibility, defining non-C99-bool BOOL as signed char" OFF)
+set(OBJC_BOOL_IS_BOOL_MODE auto CACHE STRING "Control whether BOOL should be defined as C99 bool. One of 'auto', 'always', 'never'.")
+
+if ("${OBJC_BOOL_IS_BOOL_MODE}" STREQUAL "always")
+	set(OBJC_BOOL_IS_BOOL_MODE_CONFIG OBJC_BOOL_IS_BOOL_MODE_ALWAYS)
+elseif ("${OBJC_BOOL_IS_BOOL_MODE}" STREQUAL "never")
+	set(OBJC_BOOL_IS_BOOL_MODE_CONFIG OBJC_BOOL_IS_BOOL_MODE_NEVER)
+else()
+	set(OBJC_BOOL_IS_BOOL_MODE_CONFIG OBJC_BOOL_IS_BOOL_MODE_AUTO)
+endif()
 
 # For release builds, we disable spamming the terminal with warnings about
 # selector type mismatches
 add_compile_definitions($<$<CONFIG:Release>:NO_SELECTOR_MISMATCH_WARNINGS>)
 add_compile_definitions($<$<BOOL:${TYPE_DEPENDENT_DISPATCH}>:TYPE_DEPENDENT_DISPATCH>)
 add_compile_definitions($<$<BOOL:${ENABLE_TRACING}>:WITH_TRACING=1>)
 add_compile_definitions($<$<BOOL:${DEBUG_ARC_COMPAT}>:DEBUG_ARC_COMPAT>)
-add_compile_definitions($<$<BOOL:${STRICT_APPLE_COMPATIBILITY}>:STRICT_APPLE_COMPATIBILITY>)
 
 configure_file(objc/objc-config.h.in objc/objc-config.h @ONLY)
 include_directories("${PROJECT_BINARY_DIR}/objc/")

objc/objc-config.h.in

@@ -1 +1,18 @@
 #cmakedefine STRICT_APPLE_COMPATIBILITY @STRICT_APPLE_COMPATIBILITY@
+
+#define OBJC_BOOL_IS_BOOL_MODE_AUTO 1
+#define OBJC_BOOL_IS_BOOL_MODE_ALWAYS 2
+#define OBJC_BOOL_IS_BOOL_MODE_NEVER 3
+#cmakedefine OBJC_BOOL_IS_BOOL_MODE @OBJC_BOOL_IS_BOOL_MODE_CONFIG@
+
+#define OBJC_BOOL_TYPE_STDBOOL 1
+#define OBJC_BOOL_TYPE_TRADITIONAL 2
+#define OBJC_BOOL_TYPE_APPLE 3
+
+#if OBJC_BOOL_IS_BOOL_MODE == OBJC_BOOL_IS_BOOL_MODE_ALWAYS || (defined(__OBJC_BOOL_IS_BOOL) && OBJC_BOOL_IS_BOOL_MODE == OBJC_BOOL_IS_BOOL_MODE_AUTO)
+#define OBJC_BOOL_TYPE OBJC_BOOL_TYPE_STDBOOL
+#elif STRICT_APPLE_COMPATIBILITY
+#define OBJC_BOOL_TYPE OBJC_BOOL_TYPE_APPLE
+#else
+#define OBJC_BOOL_TYPE OBJC_BOOL_TYPE_TRADITIONAL
+#endif

objc/runtime.h

@@ -44,6 +44,10 @@ extern "C" {
 #include <sys/types.h>
 #include "Availability.h"
 
+#if OBJC_BOOL_TYPE == OBJC_BOOL_TYPE_STDBOOL
+#include <stdbool.h>
+#endif
+
 // Undo GNUstep substitutions
 #ifdef class_setVersion
 #	undef class_setVersion
@@ -129,7 +133,9 @@ typedef struct objc_method *Method;
 /**
  * Objective-C boolean type.
  */
-#	ifdef STRICT_APPLE_COMPATIBILITY
+#	if OBJC_BOOL_TYPE == OBJC_BOOL_TYPE_STDBOOL
+typedef bool BOOL;
+#	elif OBJC_BOOL_TYPE == OBJC_BOOL_TYPE_APPLE
 typedef signed char BOOL;
 #	else
 #		if defined(__vxworks) || defined(_WIN32)