[fix] trying to get mingw build to work - cross-compile to windows.

[stef]

this tries to fix the mingw build, so that a shared dll can be cross compiled. a simple

you need somewhere a liboprf that has already been treated with a make mingw and then point the OPRFHOME directory in the correct location, for example when liboprf and libopaque are both in the same root, this would work:

OPRFHOME=../../liboprf/src make clean mingw64

the result should be a libopaque.dll, which according to file(1) is

libopaque.dll: PE32+ executable (DLL) (console) x86-64, for MS Windows, 20 sections

for lack of having a windows system myself, testing is up to whoever. feedback, patches very welcome.