shairport-sync: fix build when both variants are selected

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

sound/shairport-sync/Makefile

@@ -10,7 +10,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=shairport-sync
 PKG_VERSION:=2.4.1
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=git://github.com/mikebrady/shairport-sync.git
@@ -19,6 +19,8 @@ PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_MAINTAINER:=Ted Hess <thess@kitschensync.net>, Mike Brady <mikebrady@eircom.net>
 
+PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_SOURCE_SUBDIR)
+
 PKG_LICENSE:=MIT
 PKG_LICENSE_FILES:=COPYING LICENSES shairport.c