gf-server: make pgf-service work the Apache mod_fastcgi

pgf-service now appears to works with Apache mod_fastcgi (under Mac OS X) in
addition to lighttpd.

One difference appears to be that while lighttpd passes the name of the PGF
file in the SCRIPT_FILENAME variable, Apache passes it in PATH_TRANSLATED.
But perhaps this depends on exactly how the fastcgi service is configured.
The following is what I added to the Apache configuration file to test this:

	<Directory /Library/WebServer/Documents/fcgi-bin>
	  SetHandler fastcgi-script
	  Options +ExecCGI
	</Directory>
	Action pgf-service /fcgi-bin/pgf-service
	AddHandler pgf-service .pgf

src/server/PGFService.hs

@@ -33,7 +33,11 @@ logFile = "pgf-error.log"
 
 newPGFCache = newCache PGF.readPGF
 
-getPath = getVarWithDefault "SCRIPT_FILENAME" ""
+getPath =
+    do path <- getVarWithDefault "PATH_TRANSLATED" "" -- apache mod_fastcgi
+       if null path
+          then getVarWithDefault "SCRIPT_FILENAME" "" -- lighttpd
+          else return path
 
 cgiMain :: Cache PGF -> CGI CGIResult
 cgiMain cache = handleErrors . handleCGIErrors $