intl: Make the LCOV workaround non-intrusive
* gettext-runtime/intl/Makefile.in (PLURAL_OBJECT): New variable.
(OBJECTS): Refer to $(PLURAL_OBJECT) instead of plural.$lo.
(pluralx.c, pluralx.lo): New rule.
(mostlyclean): Remove pluralx.c.
* gettext-tools/configure.ac: Replace PLURAL_OBJECT in intl/Makefile.
