HAS_REMOTE_API support for Windows. Adds support for MingW too.

* HAS_REMOTE_API still defaults to 0
* When HAS_REMOTE_API=0 only libraries advapi32.lib, ole32.lib and shell32.lib are required.
* Setting HAS_REMOTE_API=1 requires curl: https://curl.haxx.se/libcurl/
*- Also need to manually install 7-Zip (http://www.7-zip.org).
*- Will automatically install Tinyxml2 (http://www.grinninglizard.com/tinyxml2/).
*- Will automatically install http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml

date.h

@@ -50,17 +50,20 @@ namespace date
 // MSVC's constexpr support is still a WIP, even in VS2015.
 // Fall back to a lesser mode to support it.
 // TODO: Remove this or retest later once MSVC's constexpr improves.
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && ! defined(__clang__)
+// MS cl compiler.
 #  define CONSTDATA const
 #  define CONSTCD11
 #  define CONSTCD14
 #  define NOEXCEPT _NOEXCEPT
 #elif __cplusplus >= 201402
+// C++14
 #  define CONSTDATA constexpr
 #  define CONSTCD11 constexpr
 #  define CONSTCD14 constexpr
 #  define NOEXCEPT noexcept
 #else
+// C++11
 #  define CONSTDATA constexpr
 #  define CONSTCD11 constexpr
 #  define CONSTCD14
@@ -3826,14 +3829,16 @@ class time_of_day
 {
     using base = detail::time_of_day_storage<Duration>;
 public:
-#ifndef _MSC_VER
+#if !(_MSC_VER && !defined(__clang__))
+    // C++11
     using base::base;
 #else
+    // MS cl compiler workaround.
     template <class ...Args>
     explicit time_of_day(Args&& ...args)
         : base(std::forward<Args>(args)...)
         {}
-#endif // _MSC_VER
+#endif
 };
 
 template <class Rep, class Period,

test/tz_test/validate.cpp

@@ -11,7 +11,6 @@ test_info(const date::time_zone* zone, const date::sys_info& info)
     auto mid = begin + (end - begin) /2;
     using sys_microseconds = sys_time<microseconds>;
     using zoned_microseconds = zoned_time<microseconds>;
-    using local_microseconds = local_time<microseconds>;
     zoned_microseconds local{zone};
 
     if (begin > sys_days{jan/1/1700})

tz.h

@@ -29,29 +29,25 @@
 
 // Get more recent database at http://www.iana.org/time-zones
 
-// Questions:
+// The notion of "current timezone" is something the operating system is expected to "just
-// 1.  Reload database.
+// know". How it knows this is system specific. It's often a value set by the user at OS
-// 4.  Is the utc to sys renaming complete?  Was it done correctly?
+// intallation time and recorded by the OS somewhere. On Linux and Mac systems the current
+// timezone name is obtained by looking at the name or contents of a particular file on
+// disk. On Windows the current timzeone name comes from the registry. In either method,
+// there is no guarantee that the "native" current timezone name obtained will match any
+// of the "Standard" names in this library's "database". On Linux, the names usually do
+// seem to match so mapping functions to map from native to "Standard" are typically not
+// required. On Windows, the names are never "Standard" so mapping is always required.
+// Technically any OS may use the mapping process but currently only Windows does use it.
 
-/*
+#if _WIN32
-The notion of "current timezone" is something the operating system is expected
+#  ifndef TIMEZONE_MAPPING
-to "just know". How it knows this is system specific. It's often a value
+#    define TIMEZONE_MAPPING 1
-set by the user at OS intallation time and recorded by the OS somewhere.
+#  endif
-On Linux and Mac systems the current timezone name is obtained by looking at
+#else
-the name or contents of a particular file on disk.
+#  if TIMEZONE_MAPPING
-On Windows the current timzeone name comes from the registry.
+#    error "Timezone mapping is not required or not implemented for this platform."
-In either method, there is no guarantee that the "native" current timezone name obtained
+#  endif
-will match any of the "Standard" names in this library's "database".
-On Linux, the names usually do seem to match so mapping functions to map from
-native to "Standard" are typically not required.
-On Windows, the names are never "Standard" so mapping is always required.
-Technically any OS may use the mapping process but currently only Windows does use it.
-*/
-
-#ifdef _WIN32
-#ifndef TIMEZONE_MAPPING
-#define TIMEZONE_MAPPING 1
-#endif
 #endif
 
 #ifndef LAZY_INIT
@@ -59,10 +55,10 @@ Technically any OS may use the mapping process but currently only Windows does u
 #endif
 
 #ifndef HAS_REMOTE_API
-#  ifndef _MSC_VER
+#  if _WIN32
-#    define HAS_REMOTE_API 1
-#  else
 #    define HAS_REMOTE_API 0
+#  else
+#    define HAS_REMOTE_API 1
 #  endif
 #endif
 
@@ -735,10 +731,10 @@ bool        remote_install(const std::string& version);
 
 const time_zone* locate_zone(const std::string& tz_name);
 #ifdef TZ_TEST
-#ifdef _WIN32
+#  if _WIN32
 const time_zone* locate_native_zone(const std::string& native_tz_name);
-#endif
+#  endif // _WIN32
-#endif
+#endif // TZ_TEST
 const time_zone* current_zone();
 
 // zoned_time