cmake: turn MODULE_INSTALL_LIB_DIR into a CMake cache entry

It lets packagers/users override the module install directory via
-DMODULE_INSTALL_LIB_DIR=... while keeping the same default path.

Signed-off-by: Matěj Cepl <mcepl@cepl.eu>

Author: mcepl

CMakeLists.txt

@@ -278,7 +278,8 @@ if (NOT LUA)
     if (WIN32)
       set(MODULE_INSTALL_LIB_DIR "${CMAKE_INSTALL_PREFIX}/lib")
     else (WIN32)
-      set(MODULE_INSTALL_LIB_DIR "${CMAKE_INSTALL_PREFIX}/lib/lua/${LUA_VERSION_MAJOR}.${LUA_VERSION_MINOR}")
+      set(MODULE_INSTALL_LIB_DIR "${CMAKE_INSTALL_PREFIX}/lib/lua/${LUA_VERSION_MAJOR}.${LUA_VERSION_MINOR}"
+        CACHE PATH "Installation directory for module")
     endif (WIN32)
   endif (BUILD_MODULE)
   if (BUILD_STATIC_LIBS)