I do:

# make DESTDIR=/usr/local/src/BUILDDIR build > build.out 2>&1

And I get:

...
cc -O  -Werror -Wall -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith  -DNLS -DYP -DLIBC_SCCS -DSYSLIBC_SCCS -D_REENTRANT -I/usr/src/lib/libc/include -D__DBINTERFACE_PRIVATE -DRESOLVSORT -DPOSIX_MISTAKE -DFLOATING_POINT -nostdinc -idirafter /usr/local/src/BUILDDIR/usr/include -c /usr/src/lib/libc/gen/err.c
/usr/src/lib/libc/gen/err.c:51: stdarg.h: No such file or directory
*** Error code 1

Stop.
*** Error code 1

Sure enough, there's no stdarg.h installed into $(DESTDIR)/usr/include.
I see two sources of a stdarg.h, one in /usr/src/sys/arch/i386/include/stdarg.h
and one in /usr/src/gnu/dist/gcc/ginclude/stdarg.h.  The former was
installed into $(DESTDIR)/usr/include/i386, but nothing got installed
in <stdarg.h>.  Is there some Makefile which I should suspect of being out
of date?  Has the build tree come to depend on EGCS without actually making