Do not set OUTPUT_DIRECTORY if it is already set

This can happen in unit tests where a test has TARGET set to "../name",
which requires the target to placed in the parent binary directory.

It is possible to run into a second assignment for OUTPUT_DIRECTORY via
the DESTDIR property (e.g: qdbushmarshall test) which can then result in
two OUTPUT_DIRECTORY values.

However, the first one needs to take precedence or the tests won't
execute properly.

Change-Id: Ib263843fa86c3dd68d92a0989b95f2890335c92d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

util/cmake/pro2cmake.py

@@ -2710,6 +2710,12 @@ def write_main_part(
     # Check for DESTDIR override
     destdir = scope.get_string("DESTDIR")
     if destdir:
+        already_added = False
+        for line in extra_lines:
+            if line.startswith("OUTPUT_DIRECTORY"):
+                already_added = True
+                break
+        if not already_added:
             destdir = replace_path_constants(destdir, scope)
             extra_lines.append(f'OUTPUT_DIRECTORY "{destdir}"')