Fix PBX_MODDIR setting when compiling directly against the asterisk source directory (where lib/asterisk/modules directory will not exist) (Reported by Nikola Ciprich)

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4629 43c1dc8c-776e-4df2-8ef1-6e829299be21