Fix build on MinGW/Cygwin when -std=c++nn is passed

That flag forces ISO conformance removing the declarations of the
required _w* functions.
Move the implementations using those into source files and undef
__STRICT_ANSI__ inside that file

Also allows to not use the *_s functions on MSVC as
_CRT_SECURE_NO_WARNINGS can be safely define inside the cpp files.

src/cstdlib.cpp

@@ -0,0 +1,121 @@
+//
+//  Copyright (c) 2012 Artyom Beilis (Tonkikh)
+//  Copyright (c) 2020 Alexander Grund
+//
+//  Distributed under the Boost Software License, Version 1.0. (See
+//  accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#define BOOST_NOWIDE_SOURCE
+
+#ifdef _MSC_VER
+#define _CRT_SECURE_NO_WARNINGS
+#elif(defined(__MINGW32__) || defined(__CYGWIN__)) && defined(__STRICT_ANSI__)
+// Need the _w* functions which are extensions on MinGW/Cygwin
+#undef __STRICT_ANSI__
+#endif
+
+#include <boost/nowide/cstdlib.hpp>
+
+#if !BOOST_WINDOWS
+namespace boost {
+namespace nowide {
+    int setenv(const char* key, const char* value, int overwrite)
+    {
+        return ::setenv(key, value, overwrite);
+    }
+
+    int unsetenv(const char* key)
+    {
+        return ::unsetenv(key);
+    }
+
+    int putenv(char* string)
+    {
+        return ::putenv(string);
+    }
+} // namespace nowide
+} // namespace boost
+#else
+#include <boost/nowide/stackstring.hpp>
+#include <vector>
+#include <windows.h>
+
+namespace boost {
+namespace nowide {
+    char* getenv(const char* key)
+    {
+        static stackstring value;
+
+        const wshort_stackstring name(key);
+
+        static const size_t buf_size = 64;
+        wchar_t buf[buf_size];
+        std::vector<wchar_t> tmp;
+        wchar_t* ptr = buf;
+        size_t n = GetEnvironmentVariableW(name.get(), buf, buf_size);
+        if(n == 0 && GetLastError() == 203) // ERROR_ENVVAR_NOT_FOUND
+            return 0;
+        if(n >= buf_size)
+        {
+            tmp.resize(n + 1, L'\0');
+            n = GetEnvironmentVariableW(name.get(), &tmp[0], static_cast<unsigned>(tmp.size() - 1));
+            // The size may have changed
+            if(n >= tmp.size() - 1)
+                return 0;
+            ptr = &tmp[0];
+        }
+        value.convert(ptr);
+        return value.get();
+    }
+
+    int setenv(const char* key, const char* value, int overwrite)
+    {
+        const wshort_stackstring name(key);
+        if(!overwrite)
+        {
+            wchar_t unused[2];
+            if(GetEnvironmentVariableW(name.get(), unused, 2) != 0 || GetLastError() != 203) // ERROR_ENVVAR_NOT_FOUND
+                return 0;
+        }
+        const wstackstring wval(value);
+        if(SetEnvironmentVariableW(name.get(), wval.get()))
+            return 0;
+        return -1;
+    }
+
+    int unsetenv(const char* key)
+    {
+        const wshort_stackstring name(key);
+        if(SetEnvironmentVariableW(name.get(), 0))
+            return 0;
+        return -1;
+    }
+
+    int putenv(char* string)
+    {
+        const char* key = string;
+        const char* key_end = string;
+        while(*key_end != '=' && *key_end != '\0')
+            key_end++;
+        if(*key_end == '\0')
+            return -1;
+        const wshort_stackstring wkey(key, key_end);
+        const wstackstring wvalue(key_end + 1);
+
+        if(SetEnvironmentVariableW(wkey.get(), wvalue.get()))
+            return 0;
+        return -1;
+    }
+
+    int system(const char* cmd)
+    {
+        if(!cmd)
+            return _wsystem(0);
+        const wstackstring wcmd(cmd);
+        return _wsystem(wcmd.get());
+    }
+} // namespace nowide
+} // namespace boost
+#endif