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/DtSearch/Makefile.am

@@ -9,7 +9,7 @@ CLEANFILES = boolyac.h boolyac.c
 AM_YFLAGS = -d
 
 libDtSearch_la_CFLAGS = -DI18N_MSG -DMULTIBYTE -I./raima \
-			-I$(top_builddir)/include/Dt
+			@DT_INCDIR@
 
 
 libDtSearch_la_LIBADD = raima/libraima.la

cde/lib/DtSearch/raima/Makefile.am

@@ -5,7 +5,7 @@ noinst_LTLIBRARIES = libraima.la
 libraima_la_LIBADD = -lc -lm
 
 libraima_la_CFLAGS = -DCDE_INSTALLATION_TOP=\"${prefix}\" \
-		     -I$(top_builddir)/include/Dt \
+		     @DT_INCDIR@ \
 		     -DCDE_CONFIGURATION_TOP=\"$(CDE_CONFIGURATION_TOP)\" \
 		     -DNO_TRANS -DUNIX -D_POSIX_SOURCE -DMULTIBYTE