Added a test using winmain.

test cases/windows/2 winmain/meson.build

@@ -0,0 +1,4 @@
+project('winmain', 'c')
+
+exe = executable('prog', 'prog.c')
+add_test('winmain', exe)

test cases/windows/2 winmain/prog.c

@@ -0,0 +1,10 @@
+#include<windows.h>
+
+int APIENTRY
+WinMain(
+    HINSTANCE hInstance,
+    HINSTANCE hPrevInstance,
+    LPSTR lpszCmdLine,
+    int nCmdShow) {
+    return 0;
+}