We don't have libboost_context.so, just libboost_context-mt.so (-mt stands
for multithreading).

--- a/src/libutil/local.mk
+++ b/src/libutil/local.mk
@@ -6,7 +6,7 @@
 
 libutil_SOURCES := $(wildcard $(d)/*.cc)
 
-libutil_LDFLAGS = $(LIBLZMA_LIBS) -lbz2 -pthread $(OPENSSL_LIBS) $(LIBBROTLI_LIBS) -lboost_context
+libutil_LDFLAGS = $(LIBLZMA_LIBS) -lbz2 -pthread $(OPENSSL_LIBS) $(LIBBROTLI_LIBS) -lboost_context-mt
 
 libutil_LIBS = libformat