Enable configure tests for system libmng and libtiff

The current code didn't run the tests anymore, but was still
based on the assumption that qtbase/configure set up the
right configuration.

Clean up the pro files for mng and tiff while we're at it.

Change-Id: Iee46c183fda57ec07b60a80089af7925db959c40
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

qtimageformats.pro

+load(configure)
+qtCompileTest(libmng)
+qtCompileTest(libtiff)
+
 load(qt_parts)
 
 include(doc/doc.pri)

src/plugins/imageformats/mng/mng.pro

@@ -4,9 +4,9 @@ load(qt_plugin)
 HEADERS += qmnghandler_p.h
 SOURCES += qmnghandler.cpp
 OTHER_FILES += mng.json
-contains(QT_CONFIG, system-mng) {
-        if(unix|win32-g++*):LIBS += -lmng
-        else:win32:         LIBS += libmng.lib
+config_libmng {
+    unix|win32-g++*: LIBS += -lmng
+    else:win32: LIBS += libmng.lib
 } else {
     include($$PWD/../../../3rdparty/libmng.pri)
     *-g++*: QMAKE_CFLAGS_WARN_ON += -Wno-unused-parameter

src/plugins/imageformats/tiff/qtiffhandler.pri

-# common to plugin and built-in forms
-INCLUDEPATH *= $$PWD
-HEADERS += $$PWD/qtiffhandler_p.h
-SOURCES += $$PWD/qtiffhandler.cpp
-contains(QT_CONFIG, system-tiff) {
-        if(unix|win32-g++*):LIBS += -ltiff
-        else:win32:         LIBS += libtiff.lib
-} else {
-    include($$PWD/../../../3rdparty/libtiff.pri)
-}

src/plugins/imageformats/tiff/tiff.pro

 TARGET  = qtiff
 load(qt_plugin)
 
-include(qtiffhandler.pri)
-SOURCES += main.cpp
+HEADERS += qtiffhandler_p.h
+SOURCES += main.cpp qtiffhandler.cpp
 OTHER_FILES += tiff.json
 
+config_libtiff {
+    unix|win32-g++*: LIBS += -ltiff
+    else:win32: LIBS += libtiff.lib
+} else {
+    include($$PWD/../../../3rdparty/libtiff.pri)
+}
+
 DESTDIR = $$QT.gui.plugins/imageformats
 target.path += $$[QT_INSTALL_PLUGINS]/imageformats
 INSTALLS += target