Makefile.flags: survive a build system which has no pthread

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Makefile.flags b/Makefile.flags
index bb90a08..b29b068 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -121,11 +121,16 @@
 # fall back to using a temp file:
 CRYPT_AVAILABLE := $(shell echo 'int main(void){return 0;}' >crypttest.c; $(CC) $(CFLAGS) -lcrypt -o /dev/null crypttest.c >/dev/null 2>&1 && echo "y"; rm crypttest.c)
 ifeq ($(CRYPT_AVAILABLE),y)
-LDLIBS += m crypt pthread
+LDLIBS += m crypt
 else
 LDLIBS += m
 endif
 
+PTHREAD_AVAILABLE := $(shell echo 'int main(void){return 0;}' >pthreadtest.c; $(CC) $(CFLAGS) -lpthread -o /dev/null pthreadtest.c >/dev/null 2>&1 && echo "y"; rm pthreadtest.c)
+ifeq ($(PTHREAD_AVAILABLE),y)
+LDLIBS += pthread
+endif
+
 ifeq ($(CONFIG_PAM),y)
 # libpam uses libpthread, libdl and libaudit, so for static builds busybox
 # must be linked to libpthread, libdl and libaudit. On some platforms that