Remove "convenience header" "gettext.h" and use <libintl.h> instead.

gettextize provides a local file called "gettext.h".  Remove this and
use <libintl.h> from glibc headers instead.

Most of this change is mechanical: #include <libintl.h> in every C
file which uses any gettext function.  But also we remove the
gettext.h file, and adjust the "_" macros.

Note that this effectively removes the ./configure --disable-nls
option, although we don't know if that ever worked.

fish/rc.c

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <libintl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/un.h>