Add support for the Go programming language.

Add support for the Go programming language.

* lib/autoconf/go.m4: New file.
* lib/autoconf/autoconf.m4: Include autoconf/go.m4.
* lib/autoconf/Makefile.am (dist_autoconflib_DATA): Add go.m4.
* lib/freeze.mk (autoconf_m4f_dependencies): Add
$(src_libdir)/autoconf/go.m4.
* doc/autoconf.texi: Rebuild menus.
(Preset Output Variables): Mention Go.  Document GOFLAGS.
(Libraries): Mention Go.
(Go Compiler): New subsection.
(Language Choice): Mention Go.
(Generating Sources): Likewise.
(Running the Preprocessor): Likewise.
* tests/go.at: New file.
* tests/suite.at: Include go.at and acgo.at.
* tests/local.at (_AT_CHECK_ENV): Add GOC and GOFLAGS.
* tests/Makefile.am (TESTSUITE_GENERATED_AT): Add
$(srcdir)/acgo.at.
(TESTSUITE_HAND_AT): Add go.at.
(AUTOCONF_FILES): Add $(autoconfdir)/go.m4.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>