Move config adjustment to config_adjust

After this change, check_config.h does not have any #defined except:
- the standard header double-inclusion guard
- short-lived helpers that are #undef-ed in the same paragraph

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 1dcd903..1741d8b 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -27,18 +27,8 @@
 #if !defined(MBEDTLS_PLATFORM_C)
 #error "MBEDTLS_PLATFORM_C is required on Windows"
 #endif
-
-/* Fix the config here. Not convenient to put an #ifdef _WIN32 in mbedtls_config.h as
- * it would confuse config.py. */
-#if !defined(MBEDTLS_PLATFORM_SNPRINTF_ALT) && \
-    !defined(MBEDTLS_PLATFORM_SNPRINTF_MACRO)
-#define MBEDTLS_PLATFORM_SNPRINTF_ALT
-#endif
-
-#if !defined(MBEDTLS_PLATFORM_VSNPRINTF_ALT) && \
-    !defined(MBEDTLS_PLATFORM_VSNPRINTF_MACRO)
-#define MBEDTLS_PLATFORM_VSNPRINTF_ALT
-#endif
+/* See auto-enabling SNPRINTF_ALT and VSNPRINTF_ALT
+ * in * config_adjust_legacy_crypto.h */
 #endif /* _MINGW32__ || (_MSC_VER && (_MSC_VER <= 1900)) */
 
 #if defined(TARGET_LIKE_MBED) && defined(MBEDTLS_NET_C)
diff --git a/include/mbedtls/config_adjust_legacy_crypto.h b/include/mbedtls/config_adjust_legacy_crypto.h
index 696266c..6126a1e 100644
--- a/include/mbedtls/config_adjust_legacy_crypto.h
+++ b/include/mbedtls/config_adjust_legacy_crypto.h
@@ -22,6 +22,22 @@
 #ifndef MBEDTLS_CONFIG_ADJUST_LEGACY_CRYPTO_H
 #define MBEDTLS_CONFIG_ADJUST_LEGACY_CRYPTO_H
 
+/* Ideally, we'd set those as defaults in mbedtls_config.h, but
+ * putting an #ifdef _WIN32 in mbedtls_config.h would confuse config.py.
+ *
+ * So, adjust it here.
+ * Not related to crypto, but this is the bottom of the stack. */
+#if defined(__MINGW32__) || (defined(_MSC_VER) && _MSC_VER <= 1900)
+#if !defined(MBEDTLS_PLATFORM_SNPRINTF_ALT) && \
+    !defined(MBEDTLS_PLATFORM_SNPRINTF_MACRO)
+#define MBEDTLS_PLATFORM_SNPRINTF_ALT
+#endif
+#if !defined(MBEDTLS_PLATFORM_VSNPRINTF_ALT) && \
+    !defined(MBEDTLS_PLATFORM_VSNPRINTF_MACRO)
+#define MBEDTLS_PLATFORM_VSNPRINTF_ALT
+#endif
+#endif /* _MINGW32__ || (_MSC_VER && (_MSC_VER <= 1900)) */
+
 /* Auto-enable CIPHER_C when any of the unauthenticated ciphers is builtin
  * in PSA. */
 #if defined(MBEDTLS_PSA_CRYPTO_C) && \