make installkernel breaks

Greetings...
this is the deal:
I have 1 build server + dozens of servers to be upgrade to 5.3b7

all servers including the build server was 5.1.
I upgraded the build server to 5.3 beta 6... everything was fine. Then 
upgraded it to 5.3 beta 7. All smooth again.

Now, I rsync'ed the /usr/obj and /usr/src from the build server to the other 
servers (which are still 5.1), and when I run make installkernel I get the 
error below:

-------
# make installkernel KERNCONF=CUSTOM-DUAL-FW
"/usr/src/Makefile.inc1", line 830: warning: String comparison operator 
should be either == or !=
"/usr/src/Makefile.inc1", line 830: Malformed conditional 
((!defined(NO_RESCUE) ||  defined(RELEASEDIR)) &&  (${TARGET_ARCH} != 
${MACHINE_ARCH} || ${BOOTSTRAPPING} < 501101))
"/usr/src/Makefile.inc1", line 830: Missing dependency operator
"/usr/src/Makefile.inc1", line 832: if-less endif
"/usr/src/Makefile.inc1", line 832: Need an operator
make: fatal errors encountered -- cannot continue
*** Error code 1

Stop in /usr/src.
-------

Any ideas? I don't want to downgrade the build server to 5.1, then upgrade 
it to beta 7, so I can upgrade the other servers... there has to be a better 
solution.

thanks