CMake: Fix .prl generation and installation for plugins

The referenced Qt libraries had paths to the build directory, instead
of the $$[QT_INSTALL_LIBS]/ prefix. The reason was two-fold:

1. QT_BUILD_LIBDIR had the wrong value, namely
"${QT_BUILD_DIR}/platforms/qfoo".

2. The QtFinishPrlFile.cmake script was called with a wrong OUT_FILE
parameter, placing the final .prl file in the build lib dir.

As drive-by change, surround arguments for QtFinishPrlFile.cmake that
can contain spaces with double quotes.
This amends 8c8c0f65e32.

Task-number: QTBUG-84781
Change-Id: I7188b799716576b3296ee7b2d460489867b9967a
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtBuild.cmake

@@ -3008,14 +3008,14 @@ QMAKE_PRL_LIBS_FOR_CMAKE = ${prl_libs}
     )
 
     # Add a custom command that prepares the .prl file for installation
-    qt_path_join(qt_build_libdir ${QT_BUILD_DIR} ${install_dir})
-    qt_path_join(prl_file_path "${qt_build_libdir}" "${prl_file_name}")
+    qt_path_join(prl_file_path "${QT_BUILD_DIR}/${install_dir}" "${prl_file_name}")
     set(library_suffixes ${CMAKE_SHARED_LIBRARY_SUFFIX} ${CMAKE_STATIC_LIBRARY_SUFFIX})
     add_custom_command(
         TARGET ${target} POST_BUILD
-        COMMAND ${CMAKE_COMMAND} -DIN_FILE=${prl_file_name} -DOUT_FILE=${prl_file_path}
+        COMMAND ${CMAKE_COMMAND} "-DIN_FILE=${prl_file_name}"
+                "-DOUT_FILE=${prl_file_path}"
                 "-DLIBRARY_SUFFIXES=${library_suffixes}"
-                -DQT_BUILD_LIBDIR=${qt_build_libdir}
+                "-DQT_BUILD_LIBDIR=${QT_BUILD_DIR}/${INSTALL_LIBDIR}"
                 -P "${QT_CMAKE_DIR}/QtFinishPrlFile.cmake"
         VERBATIM
         )