Permit application makefile to override USE_LIBDL
Permit application makefile to override USE_LIBDL

Use ?= instead of = in assignment of USE_LIBDL so application can override value if desired.

Also fix spelling error "falgs" to "flags"
chuckatkins/legion,StanfordLegion/legion,StanfordLegion/legion,sdalton1/legion,StanfordLegion/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,chuckatkins/legion,sdalton1/legion,sdalton1/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,StanfordLegion/legion,StanfordLegion/legion,chuckatkins/legion