Maintain the existing Solaris libXt ABI, by setting the library so-version to 4
(libXt.so.4) and using the mapfile to control & version exported symbols.

Not suitable for upstream.

--- libXt-1.1.5/src/Makefile.am
+++ libXt-1.1.5/src/Makefile.am
@@ -74,7 +74,8 @@ DISTCLEANFILES = StringDefs.c
 
 libXt_la_LIBADD = @XT_LIBS@
 
-libXt_la_LDFLAGS = -version-number 6:0:0 -no-undefined
+libXt_la_LDFLAGS = -version-number 4:0:0 -no-undefined \
+	-Wl,-M,$(top_srcdir)/../mapfile-vers -lc
 
 #
 # The util directory contains a program that builds some of the sources.