abuild: don't cd to $builddir unless patches exists

some packages don't have patches. We should not error if builddir is not
set in this case.

abuild.in

@@ -530,8 +530,21 @@ getpkgver() {
 	fi
 }
 
+have_patches() {
+	local i
+	for i in $source; do
+		case "$i" in
+			*.patch) return 0;;
+		esac
+	done
+	return 1
+}
+
 default_prepare() {
 	local i
+	if ! have_patches; then
+		return 0
+	fi
 	cd "${builddir:-$srcdir/$pkgname-$pkgver}" \
 		|| { error "Is \$builddir set correctly?"; return 1; }
 	for i in $source; do