Allow building with MinGW on Windows.

README.md

@@ -111,7 +111,13 @@ and point cmake-gui to the source tree and that directory. Press
 "Configure" and "Generate", then open `build\solvespace.sln` with
 Visual C++ and build it.
 
+Alternatively it is possible to build SolveSpace using [MinGW][mingw].
+Run cmake-gui as described above but after pressing "Configure" select
+the "MSYS Makefiles" generator. After that, run `make` in the `build`
+directory; make sure that the MinGW compiler is in your `PATH`.
+
 [cmakewin]: http://www.cmake.org/download/#latest
+[mingw]: http://www.mingw.org/
 
 License
 -------

src/win32/w32main.cpp

@@ -693,8 +693,13 @@ int64_t SolveSpace::GetMilliseconds(void)
 
 int64_t SolveSpace::GetUnixTime(void)
 {
+#ifdef __MINGW32__
+    time_t ret;
+    time(&ret);
+#else
     __time64_t ret;
     _time64(&ret);
+#endif
     return (int64_t)ret;
 }