tests: skip tst_displaylabelgroups() if MLITE is not available

This is a workaround for issue #5.

We also remove the `CONFIG += link_pkgconfig` from `engine.pro` because
this directive is already set in `config.pri`.

config.pri

@@ -2,3 +2,7 @@ CONFIG += \
     c++11 \
     link_pkgconfig
 PKGCONFIG += Qt5Contacts
+
+packagesExist(mlite5) {
+    DEFINES += HAS_MLITE
+}

src/engine/engine.pro

@@ -12,10 +12,9 @@ CONFIG += plugin hide_symbols
 PLUGIN_TYPE=contacts
 DESTDIR=$${PLUGIN_TYPE}
 
-CONFIG += link_pkgconfig
 packagesExist(mlite5) {
     PKGCONFIG += mlite5
-    DEFINES += HAS_MLITE
+    # The `DEFINES` directive is already set in `config.pri`
 } else {
     warning("mlite not available. Display label groups will be generated from last name.")
 }

tests/auto/displaylabelgroups/test/tst_displaylabelgroups.cpp

@@ -139,6 +139,9 @@ void tst_DisplayLabelGroups::cleanup()
 
 void tst_DisplayLabelGroups::testDisplayLabelGroups()
 {
+#ifndef HAS_MLITE
+    QSKIP("Test has wrong expectations if MLITE is not available");
+#endif
     // this test relies on the display label grouping
     // semantics provided by the testdlggplugin.