this used to be acceptable:

mk.conf:NOGCCERROR=yes

it is not documented in BUILDING and when building lib/csu/i386_elf it
becomes "yes" in the invocation of the compiler, obviously producing an
error about a non-existent file.

it appears that this is not evaluated until after COMPILE.c:

Global:CFLAGS = ${DBG} ${COPTS} ${NOGCCERROR:U-Werror} ${CWARNFLAGS}

this works:
mk.conf:NOGCCERROR=

is that the way to use this variable, or should it accept "yes".