Link libstore against libc.

Index: src/libstore/local.mk
--- src/libstore/local.mk.orig
+++ src/libstore/local.mk
@@ -8,7 +8,7 @@ libstore_SOURCES := $(wildcard $(d)/*.cc $(d)/builtins
 
 libstore_LIBS = libutil
 
-libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS) $(SODIUM_LIBS) -pthread
+libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS) $(SODIUM_LIBS) -pthread -lc
 ifdef HOST_LINUX
  libstore_LDFLAGS += -ldl
 endif