GLPK 4.61: thread local storage in Windows DLL

Hello David,

the thread local storage is not correctly implemented for usage with
Windows in a DLL. See

https://msdn.microsoft.com/en-us/library/windows/desktop/ms686997

I think we should do the following:

Write a test case that demonstrates the usage of multithreading GLPK
using the pthread library.

Add a pthread emulation for Windows so we can use the same test code
with Windows and with Posix. For a minimalistic implementation see
http://sqlstat.sourceforge.net/doxygen/html/sqlthread_8h_source.html

Rewrite the allocation and freeing of the env variable according to the
Windows DLL requirements.

Best regards

Heinrich Schuchardt