mysql_config did not output -ldl (or equivalent) when needed for --libmysqld-libs,
so its output could be insufficient to build an application using the embedded
server.

LIBDL was already set in configure; it's now propagated all the way into the
relevant mysql_config scripts.

scripts/Makefile.am:
  We "manually" replace configure's constants in .sh scripts using sed,
  so AC_SUBST() alone is not good enough. Add LIBDL to the substitution
  list; we'll be needing it for mysql_config --libmysqld-libs
scripts/mysql_config.pl.in:
  Add LIBDL (usually -ldl) to output of mysql_config --libmysqld-libs (perl version)
scripts/mysql_config.sh:
  Add LIBDL (usually -ldl) to output of mysql_config --libmysqld-libs (shell version)