color: Make color.c package-neutral.
* gettext-tools/src/color.h (style_file_prepare): Add arguments.
* gettext-tools/src/color.c: Don't include relocatable.h.
(style_file_lookup): Add stylesdir_after_install argument.
(style_file_prepare): Add arguments.
* gettext-tools/src/write-catalog.c: Include relocatable.h.
(GETTEXTSTYLESDIR): New macro.
(msgdomain_list_print): Update style_file_prepare calls.
* gettext-tools/tests/init-env.in (GETTEXTSTYLESDIR): New environment variable.
