build: make libunwind linked dynamically

Not sure why this was first linked statically, as the commit that
introduced this simply had the message "restoring stuff that seems to
have been clobbered by the revert of the unintended merge to master".

Nearly all other libraries that we use are linked dynamically.

Make.inc

@@ -940,17 +940,17 @@ LIBUNWIND:=
 else
 ifeq ($(USE_SYSTEM_LIBUNWIND), 1)
 ifneq ($(OS),Darwin)
-LIBUNWIND:=-lunwind-generic -lunwind
+LIBUNWIND:=-lunwind
 # Only for linux since we want to use not yet released libunwind features
 JCFLAGS+=-DSYSTEM_LIBUNWIND
 JCPPFLAGS+=-DSYSTEM_LIBUNWIND
 endif
 else
 ifeq ($(OS),Darwin)
-LIBUNWIND:=$(build_libdir)/libosxunwind.a
+LIBUNWIND:=-losxunwind
 JCPPFLAGS+=-DLIBOSXUNWIND
 else
-LIBUNWIND:=$(build_libdir)/libunwind-generic.a $(build_libdir)/libunwind.a
+LIBUNWIND:=-lunwind
 endif
 endif
 endif

Makefile

@@ -181,6 +181,11 @@ endif
 ifeq ($(USE_LLVM_SHLIB),1)
 JL_PRIVATE_LIBS-$(USE_SYSTEM_LLVM) += libLLVM libLLVM-9jl
 endif
+ifeq ($(OS),Darwin)
+JL_PRIVATE_LIBS-$(USE_SYSTEM_LIBUNWIND) += libosxunwind
+else
+JL_PRIVATE_LIBS-$(USE_SYSTEM_LIBUNWIND) += libunwind
+endif
 
 ifeq ($(USE_SYSTEM_LIBM),0)
 JL_PRIVATE_LIBS-$(USE_SYSTEM_OPENLIBM) += libopenlibm