Ensure configure tests run when using mingw-w64

[JoshuaWierenga]

Originally all tests other than flock failed when using mingw-w64 as cc since it sticks .exe on the end of all executable names which confuses the configure script into thinking the features test binaries don't exist.

There is also a fix included for random tests not working with dev_urandom checking the host's /dev/random which exists if building on wsl and I assume cygwin despite this obviously not being the case if the resulting blink binary is run from windows directly or on msys. And rtlgenrandom using a header that doesn't exist in mingw-w64's header library, despite this, the function is available. The rtlgenrandom check could be simpler if I knew that cygwin only defined _WIN32 for native building. Otherwise I think every check has to become the mingw-w64 specific check which needs one of a few system headers to be included first.

I have tried enabling pthreads with {i686, x86_64}-w64-mingw32-gcc-10-posix but just get a windows error popup saying libwinpthread-1.dll was not found so need to investigate that.

Additionally, repeated calls to ./configure can cause some tests, mostly memccpy and __int128 to error with either "Text file busy" and at least one error but I can't reproduce it currently.

Edit: Ignore my multiple title changes, I have been up too many hours and apparently can't read my own title correctly.

[JoshuaWierenga]

Whatever macro to check for native windows and not cygwin it decided on also has to be added to random.c.