Merge #10899: [test] Qt: Use _putenv_s instead of setenv on Windows builds

0be03c7 Qt: Use _putenv_s instead of setenv on Windows builds (Brian McMichael)

Pull request description:

  Fixes https://github.com/bitcoin/bitcoin/issues/10836

  Error message I would get on `make`:
  ```
  ...
    CXXLD    bench/bench_bitcoin.exe
    OBJCXXLD qt/bitcoin-qt.exe
  qt/test/test_main.cpp: In function ‘int main(int, char**)’:
  qt/test/test_main.cpp:64:43: error: ‘setenv’ was not declared in this scope
       setenv("QT_QPA_PLATFORM", "minimal", 0);
                                             ^
  make[2]: *** [qt/test/qt_test_test_bitcoin_qt-test_main.o] Error 1
  make[2]: *** Waiting for unfinished jobs....
  make[2]: Leaving directory `/home/bmcmichael/Projects/bcoin/src'
  make[1]: *** [all-recursive] Error 1
  make[1]: Leaving directory `/home/bmcmichael/Projects/bcoin/src'
  make: *** [all-recursive] Error 1
  ```

  `setenv` function is not available from the Microsoft runtime library. Need to use `_putenv_s` instead.

  This solution tells the compiler to use `_putenv_s` on `WIN32` compilation (Note: this also works on 64-bit Windows instances.) and `setenv` everywhere else.

  I've tested builds on Windows 10 x64 and Ubuntu 16.04 with this code.

Tree-SHA512: d53c996c890e3c6f22b4f2dcca718bef9168f19a6d4a29b8ff13391bfc0c8ea9c1cd16782b47c25b156dcbdff18bb19e23bfd5f6fefb1f373c9d5454a13fc969

src/qt/test/test_main.cpp

@@ -59,7 +59,11 @@ int main(int argc, char *argv[])
     // Prefer the "minimal" platform for the test instead of the normal default
     // platform ("xcb", "windows", or "cocoa") so tests can't unintentially
     // interfere with any background GUIs and don't require extra resources.
-    setenv("QT_QPA_PLATFORM", "minimal", 0);
+    #if defined(WIN32)
+        _putenv_s("QT_QPA_PLATFORM", "minimal");
+    #else
+        setenv("QT_QPA_PLATFORM", "minimal", 0);
+    #endif
 
     // Don't remove this, it's needed to access
     // QApplication:: and QCoreApplication:: in the tests