Fix potential quoting problem in .qmake.cache on Windows.

Unlike mingw makefiles, system() actually always uses the real
host shell, so use the host OS instead of the makefiles' path
separator as the discriminator.

Change-Id: Iaba6fc76f469e75fadd9d07e0c9e1ac07016338c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

qtcreator.pro

@@ -21,7 +21,7 @@ OTHER_FILES += dist/copyright_template.txt \
     qbs/pluginspec/pluginspec.qbs
 
 qmake_cache = $$targetPath($$IDE_BUILD_TREE/.qmake.cache)
-equals(QMAKE_DIR_SEP, /): {
+!equals(QMAKE_HOST.os, Windows) {
     maybe_quote = "\""
     maybe_backslash = "\\"
 }