Add libintl link flags to SVN_INTL_LIBS on *nix builds.

The SVN_INTL_LIBS variable was referred to but never defined.

* Makefile.in: Define SVN_INTL_LIBS.

* configure.ac: Set SVN_INTL_LIBS to the linker flags required
   to link to libintl and export the value of SVN_INTL_LIBS
   to the Makefile.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577223 13f79535-47bb-0310-9956-ffa450edef68