Ignore existing build directories in Ubuntu PPA setup

Author: scheibel

deploy/ubuntu-ppa/debian/rules

@@ -12,10 +12,10 @@ clean:
 build: build-arch
 
 build-arch:
-	mkdir $(BUILDDIR)
+	mkdir -p $(BUILDDIR)
 	cd $(BUILDDIR);cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DOPTION_BUILD_TESTS=Off -DOPTION_BUILD_EXAMPLES=On -DOPTION_BUILD_DOCS=On ..
 	make -C $(BUILDDIR)
-	mkdir $(BUILDDEBUGDIR)
+	mkdir -p $(BUILDDEBUGDIR)
 	cd $(BUILDDEBUGDIR);cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX=/usr -DOPTION_BUILD_TESTS=Off -DOPTION_BUILD_EXAMPLES=Off -DOPTION_BUILD_DOCS=Off ..
 	make -C $(BUILDDEBUGDIR)