Make getenv thread-safe

In order to return a non-owning pointer without memory leaks the
function needs to use a static variable.
When calling it from multiple threads there is a data race during the
assignment (and conversion) to this variable.
Fix by making it `thread_local`.

Fixes #189

include/boost/nowide/cstdlib.hpp

@@ -21,7 +21,12 @@ namespace nowide {
     ///
     /// \brief UTF-8 aware getenv. Returns 0 if the variable is not set.
     ///
-    /// This function is not thread safe or reenterable as defined by the standard library
+    /// The string pointed to shall not be modified by the program.
+    /// This function is thread-safe as long as no other thread modifies the host environment.
+    /// However subsequent calls to this function might overwrite the string pointed to.
+    ///
+    /// Warning: The returned pointer might only be valid for as long as the calling thread is alive.
+    ///          So avoid passing it across thread boundaries.
     ///
     BOOST_NOWIDE_DECL char* getenv(const char* key);
 

src/cstdlib.cpp

@@ -52,7 +52,7 @@ namespace boost {
 namespace nowide {
     char* getenv(const char* key)
     {
-        static stackstring value;
+        thread_local stackstring value;
 
         const wshort_stackstring name(key);