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>