Allow cross origin requests to GF cloud & PGF services

By adding a header

	Access-Control-Allow-Origin: *

to the HTTP responses, web browsers are informed that it is OK to call the
services from web pages hosted on other sites.

This is apparently supported in most modern browsers, so it should no longer
be necessary to resort to JSONP.

See https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS

src/server/FastCGIUtils.hs

@@ -22,7 +22,8 @@ import System.Posix
 #endif
 
 --import Network.FastCGI
-import Network.CGI
+import Network.CGI(CGI,CGIResult,setHeader,output,outputFPS,outputError,
+                   getInput,catchCGI,throwCGI)
 
 import Text.JSON
 import qualified Codec.Binary.UTF8.String as UTF8 (encodeString)
@@ -153,9 +154,10 @@ throwCGIError :: Int -> String -> [String] -> CGI a
 throwCGIError c m t = throwCGI $ toException $ CGIError c m t
 
 handleCGIErrors :: CGI CGIResult -> CGI CGIResult
-handleCGIErrors x = x `catchCGI` \e -> case fromException e of
-                                         Nothing -> throw e
-                                         Just (CGIError c m t) -> outputError c m t
+handleCGIErrors x =
+    x `catchCGI` \e -> case fromException e of
+                         Nothing -> throw e
+                         Just (CGIError c m t) -> do setXO; outputError c m t
 
 -- * General CGI and JSON stuff
 
@@ -174,11 +176,13 @@ outputEncodedJSONP json =
 outputPNG :: BS.ByteString -> CGI CGIResult
 outputPNG x = do
        setHeader "Content-Type" "image/png"
+       setXO
        outputFPS x
 
 outputBinary :: BS.ByteString -> CGI CGIResult
 outputBinary x = do
        setHeader "Content-Type" "application/binary"
+       setXO
        outputFPS x
 
 outputHTML :: String -> CGI CGIResult
@@ -192,9 +196,12 @@ outputPlain x = do
        outputStrict $ UTF8.encodeString x
 
 outputStrict :: String -> CGI CGIResult
-outputStrict x | x == x = output x
+outputStrict x | x == x = do setXO ; output x
                | otherwise = fail "I am the pope."
 
+setXO = setHeader "Access-Control-Allow-Origin" "*"
+     -- https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS
+
 -- * General utilities
 
 splitBy :: (a -> Bool) -> [a] -> [[a]]