configure: create some convenience AC_SUBST's for the global includes

Modify the various makefiles that were doing $(top_builddir)/include/Dt
to use them like @DT_INCDIR@, @TT_INCDIR@, etc...

cde/lib/DtSvc/DtEncap/Makefile.am

@@ -3,7 +3,7 @@ MAINTAINERCLEANFILES = Makefile.in
 noinst_LTLIBRARIES = libDtEncap.la
 
 libDtEncap_la_CFLAGS = -DMULTIBYTE -I../include -I../DtUtil2 $(TIRPCINC) \
-		       -I$(top_builddir)/include/Dt \
+		       @DT_INCDIR@ \
 		       -DCDE_CONFIGURATION_TOP=\"$(CDE_CONFIGURATION_TOP)\" \
 		       -DCDE_INSTALLATION_TOP=\"${prefix}\" \
 		       -DCDE_LOGFILES_TOP=\"$(CDE_LOGFILES_TOP)/tmp\"

cde/lib/DtSvc/DtUtil1/Makefile.am

@@ -3,7 +3,7 @@ MAINTAINERCLEANFILES = Makefile.in
 noinst_LTLIBRARIES = libDtUtil1.la
 
 libDtUtil1_la_CFLAGS = -I../include $(TIRPCINC) -DMULTIBYTE \
-		       -I$(top_builddir)/include/Dt \
+		       @DT_INCDIR@ \
 		       -DCDE_INSTALLATION_TOP=\"${prefix}\" \
 		       -DCDE_CONFIGURATION_TOP=\"$(CDE_CONFIGURATION_TOP)\" 
 

cde/lib/DtSvc/DtUtil2/Makefile.am

@@ -3,7 +3,7 @@ MAINTAINERCLEANFILES = Makefile.in
 noinst_LTLIBRARIES = libDtUtil2.la
 
 libDtUtil2_la_CFLAGS = -I../include $(TIRPCINC) -DMULTIBYTE \
-			-I$(top_builddir)/include/Dt \
+			@DT_INCDIR@ \
 			-DCDE_INSTALLATION_TOP=\"${prefix}\" \
 			-DCDE_CONFIGURATION_TOP=\"$(CDE_CONFIGURATION_TOP)\" \
 			-DMULTIBYTE -DUSE_XINERAMA $(TIRPCINC)