Refactor std::put_time detection

Hopefully this will allow both clang and GCC to work on Travis.

src/gettext_boost.cpp

@@ -437,10 +437,10 @@ std::string strftime(const std::string& format, const std::tm* time)
 	std::basic_ostringstream<char> dummy;
 	dummy.imbue(get_manager().get_locale());
 	// See utils/io.hpp for explanation of this check
-#if (defined(__clang__) && !__has_include(<experimental/any>)) || (defined(__GNUC__) && __GNUC__ < 5)
-	dummy << bl::as::ftime(format) << mktime(const_cast<std::tm*>(time));
-#else
+#if HAVE_PUT_TIME
 	dummy << std::put_time(time, format.c_str());
+#else
+	dummy << bl::as::ftime(format) << mktime(const_cast<std::tm*>(time));
 #endif
 
 	return dummy.str();

src/global.hpp

@@ -62,6 +62,7 @@
 
 // Some C++11 features are not available on all supported platforms
 #if defined(_MSC_VER)
+#define HAVE_PUT_TIME 1
 // MSVC supports these starting in MSVC 2015
 #if _MSC_VER >= 1900
 #define HAVE_REF_QUALIFIERS 1
@@ -77,6 +78,10 @@
 #endif
 
 #if defined(__clang__)
+#include <ciso646> // To ensure standard library version macros are defined
+// If it's libc++, no problem. Otherwise, attempt to detect libstdc++ version (needs GCC 5.1 or higher)
+// by testing for the existence of a header added in that version.
+#define HAVE_PUT_TIME (defined(_LIBCPP_VERSION) || __has_include(<experimental/any>))
 // Clang has convenient feature detection macros \o/
 #define HAVE_REF_QUALIFIERS __has_feature(cxx_reference_qualified_functions)
 #define HAVE_INHERITING_CTORS __has_feature(cxx_inheriting_constructors)
@@ -97,7 +102,9 @@
 #endif
 
 #if defined(__GNUC__) && !defined(__clang__)
-// GCC supports two of these from 4.6 up and the others from 4.8 up
+// GCC 5 required for this
+#define HAVE_PUT_TIME (__GNUC__ >= 5)
+// GCC supports these from 4.8 up
 #define CONSTEXPR constexpr
 #define NOEXCEPT noexcept
 #define NORETURN [[noreturn]]

src/utils/io.hpp

@@ -11,13 +11,15 @@ but WITHOUT ANY WARRANTY.
 See the COPYING file for more details.
 */
 
+#include "global.hpp"
+
 // The version of libstdc++ shipped with GCC 4.x does not have put_time in the <iomanip> header
 // Thus if GCC is the compiler being used, we can simply check the compiler version.
 // However, if clang is being used, this won't work.
 // Instead, we check for the presence of the <experimental/any> header.
 // This was introduced in GCC 5.1, so it's not a perfect check, but it appears to be the best available.
 // (Boost also uses the same check internally.)
-#if (defined(__clang__) && !__has_include(<experimental/any>)) || (defined(__GNUC__) && __GNUC__ < 5)
+#if !HAVE_PUT_TIME
 
 #include <ctime>