Doc: Get rid of QT_INSTALL_HEADERS in documentation config

The use of QT_INSTALL_HEADERS caused documentation build failures
in certain configurations. Use an alternative method to pass the
required include paths to QDoc.

Task-number: QTBUG-67925
Change-Id: Ia8c25b0cfb950f43ec60a3768cd192588a5e5617
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/androidextras/doc/qtandroidextras.qdocconf

@@ -9,8 +9,7 @@ includepaths += -I . \
                 -I .. \
                 -I ../android \
                 -I ../jni \
-                -I ./QtAndroidExtras \
-                -I $QT_INSTALL_HEADERS
+                -I ./QtAndroidExtras
 
 project = QtAndroidExtras
 description = Qt Android Extras Reference Documentation

src/src.pro

@@ -3,5 +3,6 @@ android {
     SUBDIRS += androidextras jar
 } else {
     TEMPLATE = aux
+    CONFIG += force_qt
     QMAKE_DOCS = $$PWD/androidextras/doc/qtandroidextras.qdocconf
 }