For years I have ran into this error while running make installworld:
# make installworld
[...]
===> sys/boot/i386/boot2 (install)
cc -Os -fno-guess-branch-probability -fomit-frame-pointer -fno-unit-at-a-time -mno-align-long-strings -mrtd -mregparm=3 -DUSE_XREAD -DUFS1_AND_UFS2 -DFLAGS=0x80 -DSIOPRT=0x3f8 -DSIOFMT=0x3 -DSIOSPD=9600 -I/usr/src/sys/boot/i386/boot2/../../common -I/usr/src/sys/boot/i386/boot2/../btx/lib -I. -Wall -Waggregate-return -Wbad-function-cast -Wcast-align -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wshadow -Wstrict-prototypes -Wwrite-strings -Winline --param max-inline-insns-single=100 -march=i386 -ffreestanding -mpreferred-stack-boundary=2 -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float -m32 -std=gnu99 -S -o boot2.s.tmp /usr/src/sys/boot/i386/boot2/boot2.c
sed -e '/align/d' -e '/nop/d' < boot2.s.tmp > boot2.s
rm -f boot2.s.tmp
cc -m32 -c boot2.s
ld -static -N --gc-sections -nostdlib -m elf_i386_fbsd -Ttext 0x2000 -o boot2.out /usr/obj/usr/src/sys/boot/i386/boot2/../btx/lib/crt0.o boot2.o sio.o
objcopy -S -O binary boot2.out boot2.bin
btxld -v -E 0x2000 -f bin -b /usr/obj/usr/src/sys/boot/i386/boot2/../btx/btx/btx -l boot2.ldr -o boot2.ld -P 1 boot2.bin
btxld: No such file or directory
*** [boot2.ld] Error code 1
Stop in /usr/src/sys/boot/i386/boot2.
*** [realinstall] Error code 1
Most of the posts mention bad timestamps or incorrect date. I’ve always been running ntpd though and running make buildworld before make installworld.
My longterm workaround was to make -C /usr/src/sys/boot/i386 before running make installworld.
Recently this workaround stopped working for me. Looking into it more, I realized that I was applying custom patches to the src tree and then removing them before make installworld. This was changing timestamps of source files, causing a rebuild during make installworld. Changing my scripts to leave the patch applied until after everything is installed solves it.