qbs build: Use the proper way to check the Qt version

Doing this via the product condition silently switches the product off
if Qt is too old, which will leave users wondering about the reason.
Instead, use the long-supported version check functionality of the
Depends item, which will result in a clear message in case of a failure.
Also, move the check to QtcProduct. The version requirement is for all
of QtCreator, not just the plugins.

Change-Id: Id445823e0243f5ad3b7a0ccb747a90e4fb18889b
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

qbs/imports/QtcPlugin.qbs

@@ -12,9 +12,6 @@ QtcProduct {
     property var pluginRecommends: []
     property var pluginTestDepends: []
 
-    property string minimumQtVersion: "5.6.2"
-    condition: QtcFunctions.versionIsAtLeast(Qt.core.version, minimumQtVersion)
-
     targetName: QtcFunctions.qtLibraryName(qbs, name)
     destinationDirectory: qtc.ide_plugin_path
 

qbs/imports/QtcProduct.qbs

@@ -20,6 +20,7 @@ Product {
     Depends { name: "cpp" }
     Depends { name: "qtc" }
     Depends { name: product.name + " dev headers"; required: false }
+    Depends { name: "Qt.core"; versionAtLeast: "5.6.2" }
 
     Properties {
         condition: Utilities.versionCompare(Qt.core.version, "5.7") < 0
@@ -36,8 +37,6 @@ Product {
     cpp.useCxxPrecompiledHeader: useNonGuiPchFile || useGuiPchFile
     cpp.visibility: "minimal"
 
-    Depends { name: "Qt.core" }
-
     Group {
         fileTagsFilter: installTags
         qbs.install: install