raylib.h: include <stdbool.h> if available

Previously, if <raylib.h> was #included prior to another header that
defined bool, the compilation would fail.
This is e.g. the case for <perl.h> and <objc/objc.h> which both fall
back to the <stdbool.h> if C99 is available.

The following commit includes <objc/objc.h> in src/core.c, which causes
the same problem.

Avoid this by checking for C99 bool like we already do for C++'s.

src/raylib.h

@@ -354,11 +354,11 @@
 //----------------------------------------------------------------------------------
 // Structures Definition
 //----------------------------------------------------------------------------------
-#ifndef __cplusplus
 // Boolean type
-    #ifndef bool
+#if defined(__STDC__) && __STDC_VERSION__ >= 199901L
-        typedef enum { false, true } bool;
+    #include <stdbool.h>
-    #endif
+#elif !defined(__cplusplus) && !defined(bool)
+    typedef enum { false, true } bool;
 #endif
 
 // Vector2 type