cmake/options.cmake: avoid uninitialized FILESDIR_DEF when FILESDIR is empty

Author: firewave

cmake/options.cmake

@@ -145,7 +145,7 @@ string(LENGTH "${FILESDIR}" _filesdir_len)
 # override FILESDIR if it is set or empty
 if(FILESDIR OR ${_filesdir_len} EQUAL 0)
 # TODO: verify that it is an absolute path?
-    set(FILESDIR_DEF                   ${FILESDIR})
+    set(FILESDIR_DEF                   "${FILESDIR}")
 else()
     set(FILESDIR_DEF                   ${CMAKE_INSTALL_PREFIX}/share/${PROJECT_NAME} CACHE STRING "Cppcheck files directory")
 endif()