fix: Build with MinGW version after Nov 2016

src/windows_port.h

@@ -77,6 +77,7 @@ extern int GFLAGS_DLL_DECL safe_vsnprintf(char *str, size_t size,
 #  pragma warning(push)
 #  pragma warning(disable: 4996) // ignore getenv security warning
 #endif
+#if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200112L
 inline void setenv(const char* name, const char* value, int) {
   // In windows, it's impossible to set a variable to the empty string.
   // We handle this by setting it to "0" and the NUL-ing out the \0.
@@ -98,6 +99,7 @@ inline void setenv(const char* name, const char* value, int) {
       *getenv(name) = '\0';            // works when putenv() copies nameval
   }
 }
+#endif
 #ifdef _MSC_VER
 #  pragma warning(pop)
 #endif