nm: Don't set DL_LIBS to 'none required' in configure script

nm: Don't set DL_LIBS to 'none required' in configure script

This copies the AC_SEARCH_LIBS check from the main strongSwan
configure.ac.

When building networkmanager-strongswan with slibtool if fails.

  ld: cannot find none: No such file or directory
  ld: cannot find required: No such file or directory

This is because configure.ac uses AC_SEARCH_LIBS to find dlopen which
sets the value of $ac_cv_search_dlopen to 'none required' which then
gets set in DL_LIBS and passed to slibtool.

With GNU libtool it silently ignores the unknown arguments.

Gentoo issue: https://bugs.gentoo.org/914100

Closes strongswan/strongswan#2141

Signed-off-by: orbea <orbea@riseup.net>