RELENG_4 to RELENG_5_1

I'm trying to upgrade RELENG_4 to RELENG_5_1 using source and make 
buildworld compiles
a while and then stops with:

Question is if this is supported and if yes, what should I do differently?

Pete


cc -fpic -DPIC -O -pipe -mcpu=pentiumpro -DPTHREAD_KERNEL 
-I/usr/src/lib/libpthread/../libc/include 
-I/usr/src/lib/libpthread/thread  
-I/usr/src/lib/libpthread/../../include 
-I/usr/src/lib/libpthread/arch/i386/include 
-I/usr/src/lib/libpthread/sys 
-I/usr/src/lib/libpthread/../../libexec/rtld-elf -fno-builtin 
-D_LOCK_DEBUG -D_PTHREADS_INVARIANTS -Wall  -c 
/usr/src/lib/libpthread/sys/lock.c -o lock.So
building shared library libkse.so.1
thr_libc.So: In function `sigaction':
thr_libc.So(.text+0x54): multiple definition of `_sigaction'
thr_sigaction.So(.text+0x0): first defined here
thr_libc.So: In function `sigprocmask':
thr_libc.So(.text+0x34): multiple definition of `_sigprocmask'
thr_sigprocmask.So(.text+0x0): first defined here
*** Error code 1

Stop in /usr/src/lib/libpthread.
*** Error code 1

Stop in /usr/src/lib.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.