don't try to use system zlib if it's not enabled

... as that would error out unhelpfully.
but hypothetically, there could be dynamic builds of system libpng and
sqlite3 against a static zlib, so allow it. however, it's a tad
unlikely, so default to -qt-libpng when using -qt-zlib (and -qt-sqlite3
is the default anyway).

amends dab013804.

Change-Id: I74c41e8d8a7ee1ba5add395842383d176e23f142
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/gui/configure.json

@@ -161,7 +161,9 @@
                 { "libs": "-llibpng", "condition": "config.msvc" },
                 { "libs": "-lpng", "condition": "!config.msvc" }
             ],
-            "use": "zlib"
+            "use": [
+                { "lib": "zlib", "condition": "features.system-zlib" }
+            ]
         },
         "mirclient": {
             "label": "Mir client libraries",
@@ -667,6 +669,7 @@
             "label": "  Using system libpng",
             "disable": "input.libpng == 'qt'",
             "enable": "input.libpng == 'system'",
+            "autoDetect": "features.system-zlib",
             "condition": "features.png && libs.libpng",
             "output": [ "privateFeature" ]
         },

src/sql/configure.json

@@ -117,7 +117,7 @@
                 "-lsqlite3"
             ],
             "use": [
-                { "lib": "zlib", "condition": "!config.win32" }
+                { "lib": "zlib", "condition": "!config.win32 && features.system-zlib" }
             ]
         }
     },