Disable Lex and Yacc rules whenever possible if AM_MAINTAINER_MODE

Disable Lex and Yacc rules whenever possible if AM_MAINTAINER_MODE
is used and maintainer-mode disabled.
* automake.in (Automake::struct): Define nodist_specific.
Set it in languages yacc, yaccxx, lex, and lexxx.
(register_language): Default nodist_specific to 0.
(handle_single_transform): Honor nodist_specific.
* lib/am/yacc.am (am__skipyacc): Define this in maintainer mode.
(%EXT%%DERIVED-EXT%, %OBJ%): Use $(am__skipyacc) to disable these
rules when needed.
* lib/am/lex.am (am__skiplex): Define this in maintainer mode.
(%EXT%%DERIVED-EXT%, %OBJ%): Use $(am__skiplex) to disable these
rules when needed.
* tests/mmodely.test: New file.
* tests/pr204.test: Augment to check AM_MAINTAINER_MODE and nodist_
parsers.
* tests/Makefile.am (TESTS): ADd mmodely.test.
* doc/automake.texi (Yacc and Lex): Note dependence on maintainer mode.