Recognize major.minor-build as OS version number too

As follow-up to da4e7e2, understand major.minor-build syntax in
<sparkle:minimumSystemVersion> in addition to major.minor.build.

This is a forward compatiblity measure: by using this syntax, the feed
will remain compatible with versions prior to v0.8.2. The old version
will fail to scan a string using '-' and will only see major.minor
component - and so won't misinterpret the buildnumber as service pack
version.

Should only be used if compatibility with pre-0.8.2 WinSparkle is
a concern.

src/appcast.cpp

@@ -70,14 +70,25 @@ bool is_compatible_with_windows_version(Appcast item)
     VER_SET_CONDITION(dwlConditionMask, VER_MINORVERSION, VER_GREATER_EQUAL);
     VER_SET_CONDITION(dwlConditionMask, VER_BUILDNUMBER,  VER_GREATER_EQUAL);
 
-    const int parsed = sscanf(version.c_str(), "%lu.%lu.%lu",
-                              &osvi.dwMajorVersion, &osvi.dwMinorVersion, &osvi.dwBuildNumber);
+	// parse the version number as major[.minor[.build]]:
+    int parsed = sscanf(version.c_str(), "%lu.%lu.%lu",
+                        &osvi.dwMajorVersion, &osvi.dwMinorVersion, &osvi.dwBuildNumber);
     if (parsed == 0)
     {
         // failed to parse version number, ignore the value
         return true;
     }
 
+	// allow alternative format major.minor-build for compatibility with
+	// WinSparkle < 0.8.2, which only understood major.minor.servicepack. By using '-'
+    // for the build component, older versions will only parse the major.minor part,
+    // while newer WinSparkle versions will understand the full triplet:
+    if (parsed == 2 && version.find('-') != std::string::npos)
+    {
+        parsed = sscanf(version.c_str(), "%lu.%lu-%lu",
+                        &osvi.dwMajorVersion, &osvi.dwMinorVersion, &osvi.dwBuildNumber);
+    }
+
     // backwards compatibility with WinSparkle < 0.8.2 which used major.minor.sp
     // instead of major.minor.build for the version number:
     if (parsed == 3 && osvi.dwBuildNumber < 100)