build: assume stdint.h

curl requires `stdint.h` from C99, and no longer builds without it since
v8.18.0 (after dropping VS2008 support). Assume it's available, drop
feature checks.

Also:
- drop duplicate `stdint.h` includes.
- introduce internal `HAVE_UINTPTR_T`, enabled by default.
- OS400: disable `HAVE_UINTPTR_T`.
- build: keep cmake pre-fill and `cmp-config.pl` exception because cmake
  and autotools both detect `stdint.h` implicitly.

Co-authored-by: Dan Fandrich
Ref: #20405
Ref: #20384
Follow-up to 2e1a045d89 #17931

Closes #20406

lib/curl_setup.h

@@ -466,9 +466,8 @@
 #include <curl/stdcheaders.h>
 #endif
 
-#if defined(HAVE_STDINT_H) || defined(USE_WOLFSSL)
 #include <stdint.h>
-#endif
+#define HAVE_UINTPTR_T  /* assume uintptr_t is provided by stdint.h */
 
 #ifdef __DJGPP__
 /* By default, DJGPP provides this type as a version of 'unsigned long' which
@@ -477,6 +476,11 @@
 #define uint32_t unsigned int
 #endif
 
+/* Disable uintptr_t for targets known to miss it from stdint.h */
+#ifdef __OS400__
+#undef HAVE_UINTPTR_T
+#endif
+
 #include <limits.h>
 
 #ifdef _WIN32