Fix build with -no-compile-examples

configure -no-compile-examples means that the examples won't be
compiled, but processed by qmake to generate install targets. So we
shouldn't do any substitution (like it is done for CMake targets), or
extra compilers / copies (like it is done for qmltypes).

Also install the qmldir files that some qml examples need.

Fixes: QTBUG-83375
Fixes: QTBUG-83704
Change-Id: I6a9393bd914d98a5d85f4089205510e49a435842
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

mkspecs/features/qt_example_installs.prf

@@ -74,6 +74,7 @@ sourcefiles += \
 extras = \
     $$_PRO_FILE_PWD_/README \
     $$_PRO_FILE_PWD_/README.TXT \
+    $$_PRO_FILE_PWD_/qmldir \
     $$files($$_PRO_FILE_PWD_/*.pri) \
     $$replace(_PRO_FILE_, \\.pro$, .qmlproject) \
     $$replace(_PRO_FILE_, \\.pro$, .json) \
@@ -140,6 +141,9 @@ equals(TEMPLATE, app)|equals(TEMPLATE, lib) {
     SOURCES =
     OBJECTIVE_SOURCES =
     INSTALLS -= target
+    QMAKE_SUBSTITUTES =
+    QMAKE_EXTRA_COMPILERS =
+    COPIES =
 } else {
     CONFIG += relative_qt_rpath  # Examples built as part of Qt should be relocatable
 }