Improve distclean

Remove our manually generated configuration files on make distclean.

Task-number: QTBUG-60235
Change-Id: I8b851fa80e1c17ea6ede9efb6ba0d2f108026db8
Reviewed-by: Michal Klocek <michal.klocek@qt.io>

mkspecs/features/gn_generator.prf

@@ -242,3 +242,5 @@ TEMPLATE = aux
 SOURCES =
 HEADERS =
 RESOURCES =
+
+QMAKE_DISTCLEAN += $$GN_FILE

qtwebengine.pro

@@ -13,6 +13,8 @@ isPlatformSupported() {
     log(QtWebEngine will not be built.$${EOL})
 }
 
+QMAKE_DISTCLEAN += .qmake.cache
+
 OTHER_FILES = \
     tools/buildscripts/* \
     tools/scripts/* \

src/buildtools/configure_target.pro

@@ -29,3 +29,5 @@ GN_FILE = $$OUT_PWD/../toolchain/BUILD.gn
 !build_pass {
     write_file($$GN_FILE, GN_CONTENTS, append)
 }
+
+QMAKE_DISTCLEAN += $$GN_FILE