Expand NORETURN, enable C99 unconditionally on MSVC

[[noreturn]] is supported on all the compilers we support. Still need to decide
the exact minimum versions and figure out what to do with DEPRECATED and FALLTHROUGH,
so leaving those for now.

And since we require VS 2015 and up we can enable C99 unconditionally.

src/gettext_boost.cpp

@@ -142,7 +142,7 @@ namespace
 			}
 		}
 
-		NORETURN static void throw_po_error(const std::string& lang, const std::string& dom, const std::string& detail) {
+		[[noreturn]] static void throw_po_error(const std::string& lang, const std::string& dom, const std::string& detail) {
 			std::ostringstream err;
 			err << "Error opening language file for " << lang << ", textdomain " << dom
 				<< ":\n  " << detail << '\n';

src/global.hpp

@@ -16,10 +16,8 @@
 
 #ifdef _MSC_VER
 
-// Enable C99 support for VC14
-#if _MSC_VER>=1900
+// Enable C99 support
 #define STDC99
-#endif
 
 #endif //_MSC_VER
 
@@ -42,12 +40,6 @@
 
 // Some C++11 features are not available on all supported platforms
 #if defined(_MSC_VER)
-// MSVC supports these starting in MSVC 2015
-#if _MSC_VER >= 1900
-#define NORETURN [[noreturn]]
-#else
-#define NORETURN __declspec(noreturn)
-#endif
 // MSVC supports these starting in 2017?
 // Some sources claim MSVC 2015 supports them, but let's be safe...
 #if _MSC_VER >= 1910
@@ -67,16 +59,13 @@
 #include <ciso646> // To ensure standard library version macros are defined
 
 // All supported versions of clang have these
-#define NORETURN [[noreturn]]
 #define FALLTHROUGH [[clang::fallthrough]]
 // Use GCC-style attribute because the __has_cpp_attribute feature-checking macro doesn't exist in clang 3.5
 #define DEPRECATED(reason) __attribute__((deprecated(reason)))
 
 #endif
 
 #if defined(__GNUC__) && !defined(__clang__)
-// GCC supports these from 4.8 up
-#define NORETURN [[noreturn]]
 
 // Deprecated is supported from 4.9 up
 #if __GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)

src/wml_exception.hpp

@@ -80,7 +80,7 @@
  *  @param function     The function in which the test failed.
  *  @param message      The translated message to show the user.
  */
-NORETURN void throw_wml_exception(
+[[noreturn]] void throw_wml_exception(
 		  const char* cond
 		, const char* file
 		, int line