> make[2]: Leaving directory '/tmp/nix-build-optipng-0.7.6.drv-0/optipng-0.7.6/src/pngxtern'
> gcc -c  -O2 -Wall -Wextra -I../cexcept -I../zlib -I../libpng -I../opngreduc -I../pngxtern -o optipng.o optipng.c
> gcc -c  -O2 -Wall -Wextra -I../cexcept -I../zlib -I../libpng -I../opngreduc -I../pngxtern -o optim.o optim.c
> gcc -s -o optipng optipng.o optim.o bitset.o ratio.o osys.o wildargs.o ../opngreduc/libopngreduc.a ../pngxtern/libpngxtern.a ../libpng/libpng.a ../zlib/libz.a ../gifread/libgifread.a ../pnmio/libpnmio.a ../minitiff/libminitiff.a   -lm 
> ../libpng/libpng.a(pngrutil.o): In function `png_read_filter_row':
> pngrutil.c:(.text+0x13bc): undefined reference to `png_init_filter_functions_neon'
> collect2: error: ld returned 1 exit status
> make[1]: *** [Makefile:99: optipng] Error 1
> make[1]: Leaving directory '/tmp/nix-build-optipng-0.7.6.drv-0/optipng-0.7.6/src/optipng'
> make: *** [Makefile:2: all] Error 2

This seems related: https://sourceforge.net/p/libpng/bugs/230/
The bug was apparently fixed in libpng-1.6.15, and guix is still on
1.5.26. Could this be the issue?

How do I reproduce the armhf-linux build on my x86_64 system? I tried
`guix build --fallback --target=armhf-linux optipng`, but it failed with
the following error message while building binutils. Am I missing
something?

starting phase `configure'
source directory: "/tmp/guix-build-binutils-cross-armhf-linux-2.25.1.drv-0/binutils-2.25.1" (relative from build: ".")
build directory: "/tmp/guix-build-binutils-cross-armhf-linux-2.25.1.drv-0/binutils-2.25.1"
configure flags: ("CONFIG_SHELL=/gnu/store/ykzwykkvr2c80rw4l1qh3mvfdkl7jibi-bash-4.3.42/bin/bash" "SHELL=/gnu/store/ykzwykkvr2c80rw4l1qh3mvfdkl7jibi-bash-4.3.42/bin/bash" "--prefix=/gnu/store/cmk97w8d9hzab406g70b33kh3zbgllnh-binutils-cross-armhf-linux-2.25.1" "--enable-fast-install" "--build=x86_64-unknown-linux-gnu" "--target=armhf-linux" "--with-sysroot=/" "LDFLAGS=-static-libgcc" "--with-lib-path=/no-ld-lib-path" "--disable-werror" "--enable-install-libbfd" "--enable-deterministic-archives")
checking build system type... x86_64-unknown-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking target system type... Invalid configuration `armhf-linux': machine `armhf' not recognized
configure: error: /gnu/store/ykzwykkvr2c80rw4l1qh3mvfdkl7jibi-bash-4.3.42/bin/bash ./config.sub armhf-linux failed
phase `configure' failed after 0.4 seconds
builder for `/gnu/store/zrr3b4gz2gh4m4fzpilj6zw7aw8r8c1h-binutils-cross-armhf-linux-2.25.1.drv' failed with exit code 1
@ build-failed /gnu/store/zrr3b4gz2gh4m4fzpilj6zw7aw8r8c1h-binutils-cross-armhf-linux-2.25.1.drv - 1 builder for `/gnu/store/zrr3b4gz2gh4m4fzpilj6zw7aw8r8c1h-binutils-cross-armhf-linux-2.25.1.drv' failed with exit code 1
cannot build derivation `/gnu/store/10r60df8navx9fprmlzvvv55zws8xj1x-optipng-0.7.6.drv': 1 dependencies couldn't be built
guix build: error: build failed: build of `/gnu/store/10r60df8navx9fprmlzvvv55zws8xj1x-optipng-0.7.6.drv' failed