Implement conditional AC_CONFIG_FILES: AM_COND_IF.

Implement conditional AC_CONFIG_FILES: AM_COND_IF.

* automake.in (%ac_config_files_condition): New.
(scan_autoconf_config_files): Record condition if any.
(scan_autoconf_traces): Trace _AM_COND_IF, _AM_COND_ELSE,
_AM_COND_ENDIF, updating @cond_stack as appropriate.
(handle_configure): Prefix config.status rule with condition.
Check that m4 quotation is done consistently.
* m4/cond.m4 (AM_CONDITION): Define `_AM_COND_VALUE_name'
with `name' being the name of the condition, to its shell
condition.
* m4/cond-if.m4: New file.
(_AM_COND_IF, _AM_COND_ELSE, _AM_COND_ENDIF): New trace helpers.
(AM_COND_IF): New macro, implements conditionals.
* m4/Makefile.am: Adjust.
* doc/automake.texi (Requirements, Optional, Conditionals):
Document AM_COND_IF.
* NEWS: Update.
* tests/cond39.test, tests/cond40.test, tests/cond41.test,
tests/cond42.test, tests/cond43.test: New tests.
* tests/Makefile.am: Adjust.

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