Fix -Wsign-compare and -Wundef warnings from clang on Windows

These warning were initially reported by Chromium, and I've managed to
fix the Abseil testing configuration to find these warnings.

Currently CCTZ doesn't test clang on Windows.

Author: derekmauro

src/time_zone_format.cc

@@ -39,7 +39,7 @@
 #include <limits>
 #include <string>
 #include <vector>
-#if !HAS_STRPTIME
+#if !defined(HAS_STRPTIME)
 #include <iomanip>
 #include <sstream>
 #endif
@@ -52,7 +52,7 @@ namespace detail {
 
 namespace {
 
-#if !HAS_STRPTIME
+#if !defined(HAS_STRPTIME)
 // Build a strptime() using C++11's std::get_time().
 char* strptime(const char* s, const char* fmt, std::tm* tm) {
   std::istringstream input(s);

src/time_zone_name_win.cc

@@ -21,6 +21,7 @@
 
 #include <algorithm>
 #include <atomic>
+#include <cstddef>
 #include <cstdint>
 #include <limits>
 #include <string>
@@ -129,15 +130,14 @@ std::string Utf16ToUtf8(const wchar_t* ptr, size_t size) {
   }
   const int chars_len = static_cast<int>(size);
   std::string result;
-  std::int32_t len = std::max<std::int32_t>(
-      static_cast<std::int32_t>(std::min<size_t>(
-          result.capacity(), std::numeric_limits<std::int32_t>::max())),
-      1);
+  std::size_t len = std::max<std::size_t>(
+      std::min<size_t>(result.capacity(), std::numeric_limits<int>::max()), 1);
   do {
     result.resize(len);
     // TODO: Switch to std::string::data() when we require C++17 or higher.
-    len = ::WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, ptr, chars_len,
-                                &result[0], len, nullptr, nullptr);
+    len = static_cast<std::size_t>(::WideCharToMultiByte(
+        CP_UTF8, WC_ERR_INVALID_CHARS, ptr, chars_len, &result[0],
+        static_cast<int>(len), nullptr, nullptr));
   } while (len > result.size());
   result.resize(len);
   return result;
@@ -157,15 +157,14 @@ std::string GetWindowsLocalTimeZone() {
   }
 
   std::wstring result;
-  std::int32_t len = std::max<std::int32_t>(
-      static_cast<std::int32_t>(std::min<size_t>(
-          result.capacity(), std::numeric_limits<std::int32_t>::max())),
-      1);
+  std::size_t len = std::max<std::size_t>(
+      std::min<size_t>(result.capacity(), std::numeric_limits<int>::max()), 1);
   for (;;) {
     UErrorCode status = U_ZERO_ERROR;
     result.resize(len);
-    len = getTimeZoneIDForWindowsID(info.TimeZoneKeyName, -1, nullptr,
-                                    &result[0], len, &status);
+    len = static_cast<std::size_t>(
+        getTimeZoneIDForWindowsID(info.TimeZoneKeyName, -1, nullptr, &result[0],
+                                  static_cast<int>(len), &status));
     if (U_SUCCESS(status)) {
       return Utf16ToUtf8(result.data(), len);
     }