Eliminate -Wcast-qual warnings with GCC and Clang

Use const_cast, mutable, and various other changes to avoid -Wcast-qual

src/common/filename.cpp

@@ -945,7 +945,7 @@ static wxString wxCreateTempImpl(
     wxCharBuffer buf(path.fn_str());
 
     // cast is safe because the string length doesn't change
-    int fdTemp = mkstemp( (char*)(const char*) buf );
+    int fdTemp = mkstemp(const_cast<char*>(static_cast<const char*>(buf)));
     if ( fdTemp == -1 )
     {
         // this might be not necessary as mkstemp() on most systems should have