Fix build failure with bison 3.7. Revisit bison Makefile rules.
* gettext-runtime/intl/Makefile.am ($(srcdir)/plural.c): Keep and distribute the
plural.h file. Don't use --output option.
* gettext-tools/src/Makefile.am (YACC): Removed unused variable.
(po-gram-gen.c, po-gram-gen.h): Correct the #line statements in the generated
files.
(po-gram-gen2.h): Likewise.
(cldr-plural.c, cldr-plural.h): Likewise.
