Just in case someone else could use this information, I suceeded at compiling this by adding:
# define bzero(b,len) (memset((b), '\0', (len)), (void) 0)
to uClinux-dist/user/pptp-client/pptp.c.
I am still working the configuration to make a connection, but the pptp binary seems to work well.
--- uClinux-dist/user/pptp-client/pptp.c.orig 2010-01-12 15:50:12.000000000 -0500
+++ uClinux-dist/user/pptp-client/pptp.c 2010-01-12 11:09:25.000000000 -0500
@@ -30,6 +30,8 @@
static char *call_id_str = NULL;
+#define bzero(b,len) (memset((b), '\0', (len)), (void) 0)
+
/*
* because we can be run from init, it seems unwise to just
* exit quickly for errors that probably won't have resolved