cldr-plurals: Fix --enable-relocatable compilation
Compilation of the cldr-plurals tool was broken in the relocatable case
because of missing CPPFLAGS/LDFLAGS. Add the same flags used by other
tools to the makefile.
* gettext-tools/src/Makefile.am (cldr_plurals_CPPFLAGS)
(cldr_plurals_LDFLAGS): Adjust for --enable-relocatable
compilation.
