Cygwin: Use absolute path to libltp includes

Use an absolute path to libltp includes, to allow for the fact that
we'll be compiling tests in a subdirectory.

winsup/testsuite/Makefile.in

@@ -143,7 +143,7 @@ site.exp: ./config.status Makefile
 	@echo "set CFLAGS \"$(ALL_CFLAGS)\"" >> ./tmp0
 	@echo "set MINGW_CXX \"$(MINGW_CXX)\"" >> ./tmp0
 	echo "set tmpdir $(objdir)/testsuite" >> ./tmp0
-	@echo "set ltp_includes \"$(libltp_srcdir)/include\"" >> ./tmp0
+	@echo "set ltp_includes \"$(realpath $(libltp_srcdir))/include\"" >> ./tmp0
 	@echo "## All variables above are generated by configure. Do Not Edit ##" >> ./tmp0
 	@cat ./tmp0 > site.exp
 	@cat site.bak | sed \