Use the bundled zlib in QtCore, if present.

Ref. Ie986f47e00fd0c16f2ba04d27f4258a20d61b260

Change-Id: I902cd52c2f08466cf6f42cc5e1813206e2051e95
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/3rdparty/zlib_dependency.pri

@@ -1,10 +1,7 @@
 # zlib dependency satisfied by bundled 3rd party zlib or system zlib
 contains(QT_CONFIG, system-zlib) {
-    symbian:                 LIBS_PRIVATE += -llibz
-    else:if(unix|win32-g++*):LIBS_PRIVATE += -lz
-    else:                    LIBS += zdll.lib
+    if(unix|win32-g++*): LIBS_PRIVATE += -lz
+    else:                LIBS += zdll.lib
 } else {
-# We could copy the zlib sources over to this module, but for now we don't.
-#    INCLUDEPATH +=  $$PWD/zlib
-    error("zlib not available: Qt was not built with the system's zlib")
+    INCLUDEPATH += $$[QT_INSTALL_HEADERS]/QtZlib
 }

src/plugins/imageformats/imageformats.pro

@@ -1,5 +1,6 @@
 TEMPLATE = subdirs
 SUBDIRS = \
     tga \
-    wbmp
-contains(QT_CONFIG, system-zlib): SUBDIRS += mng tiff
+    wbmp \
+    mng \
+    tiff