tidy-up: curl_setup.h, curl_setup_once.h, config-win32ce.h

- drop unused `LIBIDN_REQUIRED_VERSION` macro.
  Unused since 9c91ec7781

- drop compatibility error for `CURL_WANTS_CA_BUNDLE_ENV`.
  This macro was once set by `Makefile.mk` and Watcom makefiles.
  They are no longer supported, making the compatibility message moot.

  Follow-up to 7d7346519d #1542 (2017)
  Follow-up to c2aeb1b3ba #1538 (2017)

- document last MSVC supporting the `!_MT` condition
  Ref: https://stackoverflow.com/questions/2278919/are-the-time-functions-of-msvc-thread-safe

- flatten an `#if` tree, prefer `#ifdef`.

- fix/adjust indentation, whitespace.

Closes #15967

lib/config-win32ce.h

@@ -264,11 +264,11 @@
 /* ---------------------------------------------------------------- */
 
 #ifndef UNICODE
-#  define UNICODE
+#define UNICODE
 #endif
 
 #ifndef _UNICODE
-#  define _UNICODE
+#define _UNICODE
 #endif
 
 #define CURL_DISABLE_FILE 1

lib/curl_setup.h

@@ -122,8 +122,8 @@
 #endif
 
 /* Compatibility */
-#if defined(ENABLE_IPV6)
-#  define USE_IPV6 1
+#ifdef ENABLE_IPV6
+#define USE_IPV6 1
 #endif
 
 /*
@@ -139,10 +139,8 @@
 
 #ifdef _WIN32_WCE
 #  include "config-win32ce.h"
-#else
-#  ifdef _WIN32
+#elif defined(_WIN32)
 #  include "config-win32.h"
-#  endif
 #endif
 
 #ifdef macintosh
@@ -519,11 +517,11 @@
 #endif
 
 #ifndef struct_stat
-#  define struct_stat struct stat
+#define struct_stat struct stat
 #endif
 
 #ifndef LSEEK_ERROR
-#  define LSEEK_ERROR (off_t)-1
+#define LSEEK_ERROR (off_t)-1
 #endif
 
 #ifndef SIZEOF_TIME_T
@@ -694,9 +692,9 @@
 /*
  * MSVC threads support requires a multi-threaded runtime library.
  * _beginthreadex() is not available in single-threaded ones.
+ * Single-threaded option was last available in VS2005: _MSC_VER <= 1400
  */
-
-#if defined(_MSC_VER) && !defined(_MT)
+#if defined(_MSC_VER) && !defined(_MT)  /* available in _MSC_VER <= 1400 */
 #  undef USE_THREADS_POSIX
 #  undef USE_THREADS_WIN32
 #endif
@@ -739,8 +737,6 @@
 #error "libidn2 cannot be enabled with WinIDN or AppleIDN, choose one."
 #endif
 
-#define LIBIDN_REQUIRED_VERSION "0.4.1"
-
 #if defined(USE_GNUTLS) || defined(USE_OPENSSL) || defined(USE_MBEDTLS) || \
   defined(USE_WOLFSSL) || defined(USE_SCHANNEL) || defined(USE_SECTRANSP) || \
   defined(USE_BEARSSL) || defined(USE_RUSTLS)
@@ -788,10 +784,6 @@
 #  endif
 #endif
 
-#ifdef CURL_WANTS_CA_BUNDLE_ENV
-#error "No longer supported. Set CURLOPT_CAINFO at runtime instead."
-#endif
-
 #if defined(USE_LIBSSH2) || defined(USE_LIBSSH) || defined(USE_WOLFSSH)
 #define USE_SSH
 #endif
@@ -855,7 +847,7 @@
  */
 
 #ifndef Curl_nop_stmt
-#  define Curl_nop_stmt do { } while(0)
+#define Curl_nop_stmt do { } while(0)
 #endif
 
 /*
@@ -955,7 +947,7 @@ int getpwuid_r(uid_t uid, struct passwd *pwd, char *buf,
 #define UNITTEST static
 #endif
 
-#if defined(USE_NGHTTP2)
+#ifdef USE_NGHTTP2
 #define USE_HTTP2
 #endif
 

lib/curl_setup_once.h

@@ -24,7 +24,6 @@
  *
  ***************************************************************************/
 
-
 /*
  * Inclusion of common header files.
  */