intl: Work around LCOV relative base directory resolution
* gettext-runtime/intl/Makefile.in (plural.lo): Resolve relative
source file name given with '#line' directive, when $(srcdir) !=
$(builddir).
(mostlyclean): Remove plural.c generated in the 'plural.lo' rule.
* gettext-tools/src/plural-exp.c: Include "../intl/plural.c" instead
of "../../gettext-runtime/intl/plural.c".
