provide a flag to disable the FFI function getSystemEncoding

Ignore-this: 4c6b01ddae1d86034b7d9522c188ad75
The library for querying locale isn't as easily available on Windows as it is
on Linux. This flag provides a way for people who don't need the
getSystemEncoding function to ease the build process.

darcs-hash:20121017170920-76d51-7caa5ee9897c49a46d7beeb4aca5dcedf60e3c32

System/IO/Encoding.hs

@@ -131,12 +131,19 @@ interact f = do
   line <- hGetLine stdin
   hPutStrLn stdout (f line)
 
+#ifdef SYSTEM_ENCODING
 foreign import ccall "system_encoding.h get_system_encoding"
 	get_system_encoding :: IO CString
+#endif
 
--- | Returns the encoding used on the current system.
+-- | Returns the encoding used on the current system. Currently only supported
+-- on Linux-alikes.
 getSystemEncoding :: IO DynEncoding
 getSystemEncoding = do
+#ifdef SYSTEM_ENCODING
 	enc <- get_system_encoding
 	str <- peekCString enc
-	return $ encodingFromString str
+	return $ encodingFromString str
+#else
+	error "getSystemEncoding is not supported on this platform"
+#endif

encoding.cabal

@@ -18,11 +18,15 @@ Extra-Source-Files:
   Data/CharMap/Builder.hs
   Data/Array/Static/Builder.hs
   Data/Map/Static/Builder.hs
+  system_encoding.h
+  system_encoding.c
 
 Flag splitBase
   description:	Choose the new smaller, split-up base package.
 Flag newGHC
   description: Use ghc version > 6.10
+Flag systemEncoding
+  description: Provide the getSystemEncoding action to query the locale.
 
 Library
   Build-Depends:	binary, extensible-exceptions, HaXml >= 1.22 && < 1.24
@@ -35,6 +39,8 @@ Library
   else
     Build-Depends:	base < 3
 
+  Extensions: CPP
+
   Exposed-Modules:
     Data.Encoding
     Data.Encoding.ByteSource
@@ -102,9 +108,11 @@ Library
     Data.Map.Static
     Data.Static
     Data.CharMap
-  Includes:
-    system_encoding.h
-  Install-Includes:
-    system_encoding.h
-  C-Sources:
-    system_encoding.c
+  if flag(systemEncoding)
+    Includes:
+      system_encoding.h
+    Install-Includes:
+      system_encoding.h
+    C-Sources:
+      system_encoding.c
+    CPP-Options: -DSYSTEM_ENCODING