* check if dlopen() is in libc first, then in libdl

 * check if dlopen() is in libc first, then in libdl
 * check if clock_gettime() is in libc first, then in librt

some systems have these functions in libc, or in a specific lib.
This allows to correctly set dlopen_libs and rt_libs variables.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/ecore@55821 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33