Add SK_SYSTEM_ZLIB define.

Review URL: https://codereview.appspot.com/5991067

git-svn-id: http://skia.googlecode.com/svn/trunk@3699 2bbb7eff-a529-9590-31e7-b0007b416f81

include/config/SkUserConfig.h

@@ -121,9 +121,11 @@
 
 /*  If zlib is available and you want to support the flate compression
     algorithm (used in PDF generation), define SK_ZLIB_INCLUDE to be the
-    include path.
+    include path. Alternatively, define SK_SYSTEM_ZLIB to use the system zlib
+    library specified as "#include <zlib.h>".
  */
 //#define SK_ZLIB_INCLUDE <zlib.h>
+//#define SK_SYSTEM_ZLIB
 
 /*  Define this to allow PDF scalars above 32k.  The PDF/A spec doesn't allow
     them, but modern PDF interpreters should handle them just fine.

include/core/SkPostConfig.h

@@ -82,6 +82,12 @@
     #endif
 #endif
 
+#if defined(SK_ZLIB_INCLUDE) && defined(SK_SYSTEM_ZLIB)
+    #error "cannot define both SK_ZLIB_INCLUDE and SK_SYSTEM_ZLIB"
+#elif defined(SK_ZLIB_INCLUDE) || defined(SK_SYSTEM_ZLIB)
+    #define SK_HAS_ZLIB
+#endif
+
 ///////////////////////////////////////////////////////////////////////////////
 
 #ifndef SkNEW

src/core/SkFlate.cpp

@@ -11,7 +11,7 @@
 #include "SkFlate.h"
 #include "SkStream.h"
 
-#ifndef SK_ZLIB_INCLUDE
+#ifndef SK_HAS_ZLIB
 bool SkFlate::HaveFlate() { return false; }
 bool SkFlate::Deflate(SkStream*, SkWStream*) { return false; }
 bool SkFlate::Deflate(const void*, size_t, SkWStream*) { return false; }
@@ -26,7 +26,11 @@ bool SkFlate::HaveFlate() {
 
 namespace {
 
+#ifdef SK_SYSTEM_ZLIB
+#include <zlib.h>
+#else
 #include SK_ZLIB_INCLUDE
+#endif
 
 // static
 const size_t kBufferSize = 1024;