Handle Microsoft crypt provider without prior keys

As reported in issue #40, a call to CryptAcquireContext in
SecureRandomDataProvider fails in a fresh windows install prior to any
user keys being created in AppData\Roaming\Microsoft\Crypto\RSA.

Thanks michalrames.

ChangeLog

@@ -1,5 +1,9 @@
 2015-05-24  Jay Berkenbilt  <ejb@ql.org>
 
+	* Handle Microsoft crypt provider initialization properly for case
+	where no keys have been previously created, such as in a fresh
+	Windows installation.
+
 	* Include time.h in QUtil.hh for time_t
 
 2015-02-21  Jay Berkenbilt  <ejb@ql.org>

libqpdf/SecureRandomDataProvider.cc

@@ -42,11 +42,42 @@ class WindowsCryptProvider
   public:
     WindowsCryptProvider()
     {
-        if (! CryptAcquireContext(&crypt_prov, NULL, NULL, PROV_RSA_FULL, 0))
+        if (!CryptAcquireContext(&crypt_prov,
+                                 "Container",
+                                 NULL,
+                                 PROV_RSA_FULL,
+                                 0))
+        {
+#ifdef __GNUC__
+# if ((__GNUC__ * 100) + __GNUC_MINOR__) >= 406
+#           pragma GCC diagnostic push
+#           pragma GCC diagnostic ignored "-Wold-style-cast"
+#           pragma GCC diagnostic ignored "-Wsign-compare"
+# endif
+#endif
+            if (GetLastError() == NTE_BAD_KEYSET)
+#ifdef __GNUC__
+# if ((__GNUC__ * 100) + __GNUC_MINOR__) >= 406
+#           pragma GCC diagnostic pop
+# endif
+#endif
+            {
+                if (! CryptAcquireContext(&crypt_prov,
+                                         "Container",
+                                         NULL,
+                                         PROV_RSA_FULL,
+                                         CRYPT_NEWKEYSET))
+                {
+                    throw std::runtime_error(
+                        "unable to acquire crypt context with new keyset");
+                }
+            }
+            else
             {
                 throw std::runtime_error("unable to acquire crypt context");
             }
         }
+    }
     ~WindowsCryptProvider()
     {
         // Ignore error