Fix MacOS regression: restore libdl functionality
Fix MacOS regression: restore libdl functionality

Restore libdl functionality on MacOS that was accidentally removed in
d7fda60dfb638af7f099aff280541ce4bfda8c5d for i#1541's library independence
for Linux.

Review-URL: https://codereview.appspot.com/220190043

code4bones/dynamorio,sigma-random/dynamorio,AmesianX/dynamorio,sigma-random/dynamorio,daksunt/dynamorio,code4bones/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,code4bones/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,daksunt/dynamorio,daksunt/dynamorio,sigma-random/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,bl4ckic3/dynamorio,bl4ckic3/dynamorio