xgettext: Fix a portability problem: Don't assume anything about the value of EOF.
Pinpointed by gcc 13 warning:
warning: macro "P7_EOF" is not used [-Wunused-macros]
* gettext-tools/src/x-c.c (P7_STRING_END): Remove macro.
(phase7_getc): Transform EOF to P7_EOF.
(phase5_get): Test for P7_EOF, not EOF.
* gettext-tools/src/x-vala.c (P7_STRING_END): Remove macro.
(phase7_getc): Transform EOF to P7_EOF.
(phase3_get): Test for P7_EOF, not EOF.