Two modifications:
1. The addition of 2013 to the copyright year range for every file;
2. The use of a single year range, instead of potentially multiple
year ranges, as approved by the FSF.
* Makefile.in: Add gdb-dlfcn.c and gdb-dlfcn.h to build system.
* config.in: Add new #define HAVE_LIBDL.
* configure.ac: Add check for -ldl.
* configure: Re-generated by autoconf.
* gdb-dlfcn.c: New file.
* gdb-dlfcn.h: New file.