disable install targets for non-prefix builds

one reason to do that is some users' persistence in destroying their
non-prefix builds by trying an installation.

another reason is the fact that qt.pro's relative_qt_rpath is triggered
by the presence of an install rule for the target, which is of course
not helpful when the install dir is bogus.

Task-number: QTBUG-48406
Change-Id: I75f3940be79fcb5b86e34b975b789692423c92cb
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

mkspecs/features/qt_build_config.prf

@@ -52,6 +52,12 @@ QMAKE_DIR_REPLACE_SANE = PRECOMPILED_DIR OBJECTS_DIR MOC_DIR RCC_DIR UI_DIR
     unset(modpath)
 }
 
+# Don't actually try to install anything in non-prefix builds.
+# This is much easier and safer than making every single INSTALLS
+# assignment conditional.
+!prefix_build: \
+    CONFIG += qt_clear_installs
+
 cross_compile: \
     CONFIG += force_bootstrap
 

mkspecs/features/qt_clear_installs.prf

@@ -0,0 +1,12 @@
+#
+#  W A R N I N G
+#  -------------
+#
+# This file is not part of the Qt API.  It exists purely as an
+# implementation detail.  It may change from version to version
+# without notice, or even be removed.
+#
+# We mean it.
+#
+
+INSTALLS =