Link libexpr against libc.

Index: src/libexpr/local.mk
--- src/libexpr/local.mk.orig
+++ src/libexpr/local.mk
@@ -8,7 +8,7 @@ libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(
 
 libexpr_LIBS = libutil libstore
 
-libexpr_LDFLAGS =
+libexpr_LDFLAGS = -lc
 ifdef HOST_LINUX
  libexpr_LDFLAGS += -ldl
 endif