Respect CMAKE_INSTALL_LIBDIR in installed CMake files (#3054)

Fixes #3017
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ec87832..946fc1d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -214,7 +214,7 @@
     endmacro()
   else()
     macro(spvtools_config_package_dir TARGET PATH)
-      set(${PATH} lib/cmake/${TARGET})
+      set(${PATH} ${CMAKE_INSTALL_LIBDIR}/cmake/${TARGET})
     endmacro()
   endif()