Link to unwind when using symbolizer

Summary:
Fix #673
Closes https://github.com/facebook/folly/pull/715

Reviewed By: meyering

Differential Revision: D6394731

Pulled By: yfeldblum

fbshipit-source-id: fb92828d0c633278601125dbc64cadf0cdbaa7fe

folly/Makefile.am

@@ -734,6 +734,10 @@ libfollybase_la_LDFLAGS = $(AM_LDFLAGS) -version-info $(LT_VERSION)
 libfolly_la_LIBADD = libfollybase.la
 libfolly_la_LDFLAGS = $(AM_LDFLAGS) -version-info $(LT_VERSION) -lssl
 
+if USE_SYMBOLIZER
+libfolly_la_LDFLAGS += -lunwind
+endif
+
 FingerprintTables.cpp: generate_fingerprint_tables
 	./generate_fingerprint_tables
 CLEANFILES += FingerprintTables.cpp