Choose default source extension: AM_DEFAULT_SOURCE_EXT.

Choose default source extension: AM_DEFAULT_SOURCE_EXT.

* automake.in (handle_source_transform): Accept unconditional
literal extension in AM_DEFAULT_SOURCE_EXT as override for the
default source extension `.c'.  If set, ignore the old default
source rule for libtool libraries.
* doc/automake.texi (Default _SOURCES): Document this.
* NEWS: Update.
* tests/specflg10.test: New test.
* tests/Makefile.am: Update.
Suggestion by Akim Demaille.

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