Move thread_local variable out of function scope.

MinGW seems to have a bug with function-local thread_local variables.

src/cstdlib.cpp

@@ -48,12 +48,14 @@ namespace nowide {
 #include <vector>
 #include <windows.h>
 
+namespace {
+thread_local boost::nowide::stackstring getenv_value;
+}
+
 namespace boost {
 namespace nowide {
     char* getenv(const char* key)
     {
-        thread_local stackstring value;
-
         const wshort_stackstring name(key);
 
         static const size_t buf_size = 64;
@@ -72,8 +74,8 @@ namespace nowide {
                 return 0;
             ptr = &tmp[0];
         }
-        value.convert(ptr);
-        return value.get();
+        getenv_value.convert(ptr);
+        return getenv_value.get();
     }
 
     int setenv(const char* key, const char* value, int overwrite)