The patch comes from OpenEmbedded.
Rebased for ppp-2.4.5. Dongxiao Xu <dongxiao.xu@intel.com>

diff -urN ppp-2.4.5-orig/pppd/Makefile.linux ppp-2.4.5/pppd/Makefile.linux
--- ppp-2.4.5-orig/pppd/Makefile.linux	2010-06-30 15:51:12.043682063 +0800
+++ ppp-2.4.5/pppd/Makefile.linux	2010-06-30 17:08:21.806363042 +0800
@@ -117,10 +117,10 @@
 #LIBS     += -lshadow $(LIBS)
 endif
 
-ifneq ($(wildcard /usr/include/crypt.h),)
+#ifneq ($(wildcard /usr/include/crypt.h),)
 CFLAGS  += -DHAVE_CRYPT_H=1
 LIBS	+= -lcrypt
-endif
+#endif
 
 ifdef NEEDDES
 ifndef USE_CRYPT
