build: Fix #line statements in bison-generated files.
* gettext-tools/src/plural-exp.c: Revert changes from 2014-12-03 and 2014-12-04.
* gettext-runtime/intl/Makefile.in (PLURAL_OBJECT): Remove variable.
(OBJECTS): Use plural.lo always.
($(srcdir)/plural.c): New rule.
(.y.c, pluralx.c, pluralx.lo): Remove rules.
(mostlyclean): Delete the temporary file of the plural.c rule.
(maintainer-clean): Delete plural.c.
* gettext-tools/configure.ac (intl/Makefile): Remove PLURAL_OBJECT hack.
