Use CryptGenRandom() rather than non-existent /dev/urandom on Windows. Fixes #167.

c++/src/capnp/compiler/parser.c++

@@ -28,18 +28,33 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+#if _WIN32
+#include <windows.h>
+#undef VOID
+#endif
+
 namespace capnp {
 namespace compiler {
 
 uint64_t generateRandomId() {
   uint64_t result;
 
+#if _WIN32
+  HCRYPTPROV handle;
+  KJ_ASSERT(CryptAcquireContextW(&handle, nullptr, nullptr,
+                                 PROV_RSA_FULL, CRYPT_VERIFYCONTEXT | CRYPT_SILENT));
+  KJ_DEFER(KJ_ASSERT(CryptReleaseContext(handle, 0)) {break;});
+
+  KJ_ASSERT(CryptGenRandom(handle, sizeof(result), reinterpret_cast<BYTE*>(&result)));
+
+#else
   int fd;
   KJ_SYSCALL(fd = open("/dev/urandom", O_RDONLY));
 
   ssize_t n;
   KJ_SYSCALL(n = read(fd, &result, sizeof(result)), "/dev/urandom");
   KJ_ASSERT(n == sizeof(result), "Incomplete read from /dev/urandom.", n);
+#endif
 
   return result | (1ull << 63);
 }