PKGFILE: prevent out of tree builds

With makepkg is it possible to build sources away from the PKGFILE. The
previous behavior was crash on build if this was setup. With this
patch we prevent this possibility.

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>

PKGBUILD

@@ -14,6 +14,13 @@ backup=(etc/dracut.conf)
 source=()
 md5sums=()
 
+# out of tree builds disallowed for this PKGFILE
+BUILDDIR="${PWD}"
+PKGDEST="${PWD}"
+SRCDEST=""
+SRCPKGDEST=""
+LOGDEST=""
+
 pkgver() {
   cd ..
   desc="$(git describe)"