symbol: provide an easy means to compare them in source order

* src/symtab.c (symbols_sort): New.
(user_token_number_redeclaration): Taken from here.
diagnostics: fix the order of multiple declarations reports

On

  %token FOO "foo"
  %printer {} "foo"
  %printer {} FOO

we report

  /tmp/foo.yy:2.10-11: error: %printer redeclaration for FOO
   %printer {} "foo"
            ^^
  /tmp/foo.yy:3.10-11:     previous declaration
   %printer {} FOO
            ^^

* src/symtab.c (locations_sort): New.
Use it.
* tests/input.at (Invalid Aliases): Stress the order of diagnostics.
package: install README and the like in docdir

* Makefile.am: here.
package: install the examples

Currently, we do not install the various examples extracted from the
documentation.  Let's do it, as they are useful starting points.

* configure.ac: When --enable-gcc-warnings is set, enable ENABLE_GCC_WARNINGS.
* examples/extexi: No longer issue synclines by default.
* examples/local.mk: Except if ENABLE_GCC_WARNINGS.
* examples/calc++/local.mk, examples/mfcalc/local.mk,
* examples/rpcalc/local.mk: Install the example files.