msginit: Guess plural rules from Unicode CLDR

msginit: Guess plural rules from Unicode CLDR

* gettext-tools/src/Makefile.am (noinst_PROGRAMS): Add cldr-plurals.
(install-exec-local): Install cldr-plurals.
(BUILT_SOURCES): Add cldr-plural.c and cldr-plural.h.
(cldr_plural_SOURCES): New variable.
(cldr_plural_LDADD): New variable.
* gettext-tools/src/cldr-plural-exp.h: New file.
* gettext-tools/src/cldr-plural-exp.c: New file.
* gettext-tools/src/cldr-plural.y: New file.
* gettext-tools/src/cldr-plurals.c: New file.
* gettext-tools/src/msginit.c (plural_forms): Call cldr-plurals program
if the plural rule is not defined in the code.
* gettext-tools/doc/msginit.texi: More explanations about "meta
information".
* gettext-tools/doc/gettext.texi (Plural forms): Mention Unicode CLDR
support in msginit.
* gettext-tools/tests/cldr-plurals-1: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add new tests.
* NEWS: Mention msginit change.