fix incorrect path separators coming from installed .prl files

we use qmake properties in the installed .prl files, so the paths
need to be converted to native separators before emission.

Task-number: QTBUG-46217
Change-Id: If3fb0a84488795478fc2a701271c931c62eba6aa
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

qmake/generators/win32/winmakefile.cpp

@@ -100,10 +100,13 @@ ProString Win32MakefileGenerator::fixLibFlag(const ProString &lib)
 {
     if (lib.startsWith('/')) {
         if (lib.startsWith("/LIBPATH:"))
-            return QStringLiteral("/LIBPATH:") + escapeFilePath(lib.mid(9));
+            return QLatin1String("/LIBPATH:")
+                    + escapeFilePath(Option::fixPathToTargetOS(lib.mid(9).toQString(), false));
+        // This appears to be a user-supplied flag. Assume sufficient quoting.
         return lib;
     }
-    return escapeFilePath(lib);
+    // This must be a fully resolved library path.
+    return escapeFilePath(Option::fixPathToTargetOS(lib.toQString(), false));
 }
 
 bool