Coverage and doc fixes for AC_LANG_SOURCE and AC_LANG_PROGRAM.

Coverage and doc fixes for AC_LANG_SOURCE and AC_LANG_PROGRAM.

* tests/compile.at (AC_LANG_SOURCE, AC_LANG_SOURCE(C++))
(AC_LANG_SOURCE example, AC_LANG_PROGRAM example): New tests.
* doc/autoconf.texi (Generating Sources): Add markers for tested
examples; update quoting, and update AC_INIT usage to also set
optional URL arguments.  Mention that the examples require gcc.
Prompted by report from Brian J. Murrell.

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