Fix splitting into words of the .for expression to allow for spaces in values.  Without this change, the following valid call broke due to parsing of .MAKEFLAGS in bsd.symver.mk:
Fix splitting into words of the .for expression to allow for
spaces in values.  Without this change, the following valid
call broke due to parsing of .MAKEFLAGS in bsd.symver.mk:

cd /usr/src/lib/libc && make -n DEBUG_FLAGS="-DFOO -DBAR"

Spotted by:	Igor Sysoev
Submitted by:	Maxim Dounin, ru
MFC after:	1 week

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