libXt fixes for compilation with clang-3.4

[Thomas Klausner]

This fixes all warnings that clang-3.4 reports, except for

Error.c:265:30: warning: format string is not a string literal [-Wformat-nonliteral]
            (void) fprintf (stderr, buffer,
                                    ^~~~~~
Error.c:307:40: warning: format string is not a string literal [-Wformat-nonliteral]
            (void) snprintf (message, BIGBUF, buffer,


and:
sharedlib.c:184:12: warning: unused variable 'dummy' [-Wunused-variable]
static int dummy;                       /* avoid warning from ranlib */
           ^