build: add a HOSTCXX variable pointing at the c++ compiler

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36109 3c298f89-4303-0410-b956-a3cf2f4a3e73
tools/upx: use HOSTCXX to fix build errors on mac os x

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36110 3c298f89-4303-0410-b956-a3cf2f4a3e73