build: Fix VPATH build failures with old bison when the *.y files are modified.
* gettext-runtime/intl/Makefile.am (BISON): New variable.
(YACC): Use it.
(plural.c): Don't do the post-processing if a suitable version of bison was not
found.
* gettext-tools/src/Makefile.am (BISON): New variable.
(po-gram-gen.c, cldr-plural.c): Don't do the post-processing if a suitable
version of bison was not found.
