Fix out-of-source tree builds.

The pkg-config file is generated in the build directory, so we need to
copy it from there.
diff --git a/Makefile.in b/Makefile.in
index 00573f8..002d25d 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -264,7 +264,7 @@
 	fi
 	if [ "$(pkgconfig_DATA)" != "" ]; then \
 		$(INSTALL) -d $(DESTDIR)$(pkgconfigdir); \
-		cp $(srcdir)/$(pkgconfig_DATA) $(DESTDIR)$(pkgconfigdir)/; \
+		cp $(top_builddir)/$(pkgconfig_DATA) $(DESTDIR)$(pkgconfigdir)/; \
 	fi
 
 uninstall: