Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

src/SDL_error.c

@@ -24,7 +24,7 @@
 
 #include "SDL_error_c.h"
 
-SDL_bool SDL_SetError(SDL_PRINTF_FORMAT_STRING const char *fmt, ...)
+bool SDL_SetError(SDL_PRINTF_FORMAT_STRING const char *fmt, ...)
 {
     // Ignore call if invalid format pointer was passed
     if (fmt) {
@@ -77,7 +77,7 @@ const char *SDL_GetError(void)
     }
 }
 
-SDL_bool SDL_ClearError(void)
+bool SDL_ClearError(void)
 {
     SDL_error *error = SDL_GetErrBuf(false);
 
@@ -87,7 +87,7 @@ SDL_bool SDL_ClearError(void)
     return true;
 }
 
-SDL_bool SDL_OutOfMemory(void)
+bool SDL_OutOfMemory(void)
 {
     SDL_error *error = SDL_GetErrBuf(true);