Make the code C23-compatible

Replace the enum-defined true/false constants with the default one. Also
fix-up a function definition.

Authour: Alexander Golubev (Fat-Zer) <fatzer2@gmail.com>
Bug: https://bugs.gentoo.org/944881
Source: A dedicated patch for Gentoo

--- a/libgputils/gpcoffgen.h
+++ b/libgputils/gpcoffgen.h
@@ -88,7 +88,7 @@ extern gp_reloc_t *gp_coffgen_add_reloc(gp_section_t *Section);
 #define RELOC_DISABLE_WARN              (1 << 0)
 #define RELOC_ENABLE_CINIT_WARN         (1 << 1)
 
-extern void gp_coffgen_check_relocations(const gp_object_t *Object, unsigned int Behavior);
+extern void gp_coffgen_check_relocations(const gp_object_t *Object, gp_boolean Behavior);
 
 extern gp_boolean gp_coffgen_del_reloc(gp_section_t *Section, gp_reloc_t *Relocation);
 extern const char *gp_coffgen_reloc_type_to_str(uint16_t Type);
--- a/libgputils/gptypes.h
+++ b/libgputils/gptypes.h
@@ -26,10 +26,16 @@ Boston, MA 02111-1307, USA.  */
 
 #include "stdhdr.h"
 
+/* C99 systems have <stdbool.h>. Non-C99 systems may or may not. */
+#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+#include <stdbool.h>
+typedef int gp_boolean; /* use int rather than bool to keep ABI compatible with enum; replace with bool if you are bold */
+#else
 typedef enum {
   false = (0 == 1),
   true  = (0 == 0)
 } gp_boolean;
+#endif
 
 typedef long    gp_symvalue_t;