these derivations will be built:
  /nix/store/kqwb525yy9y19chcf29mhasayzkgcbkf-btrfs-progs-4.19-armv7l-unknown-linux-gnueabihf.drv
building '/nix/store/kqwb525yy9y19chcf29mhasayzkgcbkf-btrfs-progs-4.19-armv7l-unknown-linux-gnueabihf.drv'...
unpacking sources
unpacking source archive /nix/store/3z7xbd91mpm3r1y9h8vpcwzbyhmd1lax-btrfs-progs-v4.19.tar.xz
source root is btrfs-progs-v4.19
setting SOURCE_DATE_EPOCH to timestamp 1541239698 of file btrfs-progs-v4.19/config/install-sh
patching sources
updateAutotoolsGnuConfigScriptsPhase
Updating Autotools / GNU config script to a newer upstream version: ./config/config.sub
Updating Autotools / GNU config script to a newer upstream version: ./config/config.guess
configuring
configure flags: --disable-static --prefix=/nix/store/kig6dwdh89fy65jb4lw4z6vk0fllsrry-btrfs-progs-4.19-armv7l-unknown-linux-gnueabihf --build=x86_64-unknown-linux-gnu --host=armv7l-unknown-linux-gnueabihf
checking for armv7l-unknown-linux-gnueabihf-gcc... armv7l-unknown-linux-gnueabihf-gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... yes
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether armv7l-unknown-linux-gnueabihf-gcc accepts -g... yes
checking for armv7l-unknown-linux-gnueabihf-gcc option to accept ISO C89... none needed
checking how to run the C preprocessor... armv7l-unknown-linux-gnueabihf-gcc -E
checking for grep that handles long lines and -e... /nix/store/k7sszlqy0f9ixbnjp3ghvmdis03m5qqz-gnugrep-3.1/bin/grep
checking for egrep... /nix/store/k7sszlqy0f9ixbnjp3ghvmdis03m5qqz-gnugrep-3.1/bin/grep -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking minix/config.h usability... no
checking minix/config.h presence... no
checking for minix/config.h... no
checking whether it is safe to define __EXTENSIONS__... yes
checking for armv7l-unknown-linux-gnueabihf-gcc... (cached) armv7l-unknown-linux-gnueabihf-gcc
checking whether we are using the GNU C compiler... (cached) yes
checking whether armv7l-unknown-linux-gnueabihf-gcc accepts -g... (cached) yes
checking for armv7l-unknown-linux-gnueabihf-gcc option to accept ISO C89... (cached) none needed
checking whether C compiler accepts -std=gnu90... yes
checking build system type... x86_64-unknown-linux-gnu
checking host system type... armv7l-unknown-linux-gnueabihf
checking for an ANSI C-conforming const... yes
checking for working volatile... yes
checking whether byte ordering is bigendian... no
checking for special C compiler options needed for large files... no
checking for _FILE_OFFSET_BITS value needed for large files... 64
checking for a BSD-compatible install... /nix/store/rlrf0k4z6vasy9sqswxj66cg5vmnvpsk-coreutils-8.30/bin/install -c
checking whether ln -s works... yes
checking for armv7l-unknown-linux-gnueabihf-ar... armv7l-unknown-linux-gnueabihf-ar
checking for rm... /nix/store/rlrf0k4z6vasy9sqswxj66cg5vmnvpsk-coreutils-8.30/bin/rm
checking for rmdir... /nix/store/rlrf0k4z6vasy9sqswxj66cg5vmnvpsk-coreutils-8.30/bin/rmdir
checking for openat... yes
checking for reallocarray... yes
checking for armv7l-unknown-linux-gnueabihf-pkg-config... no
checking for pkg-config... /nix/store/l6wsvyviggk0af6c829w7qsvk1ncb3r2-pkg-config-0.29.2/bin/pkg-config
configure: WARNING: using cross tools not prefixed with host triplet
checking pkg-config is at least version 0.9.0... yes
checking execinfo.h usability... yes
checking execinfo.h presence... yes
checking for execinfo.h... yes
checking for backtrace... yes
checking for backtrace_symbols_fd... yes
checking for xmlto... /nix/store/hn2imdqln90y9xl8xinrp52wl24ks0sl-xmlto-0.0.28/bin/xmlto
checking for mv... /nix/store/rlrf0k4z6vasy9sqswxj66cg5vmnvpsk-coreutils-8.30/bin/mv
checking for a sed that does not truncate output... /nix/store/kb2h4llzakvphmryvwqljvyqqnxnslvm-gnused-4.5/bin/sed
checking for asciidoc... /nix/store/37iqfzb070cr8kcxay97ld0yizgk87zr-asciidoc-8.6.9/bin/asciidoc
checking for asciidoctor... no
checking for ext2fs... yes
checking for com_err... yes
checking for reiserfscore >= 3.6.27... no
checking for FIEMAP_EXTENT_SHARED defined in linux/fiemap.h... yes
checking for blkid... yes
checking for uuid... yes
checking for zlib... yes
checking for libzstd >= 1.0.0... yes
checking for a Python interpreter with version >= 3.4... python
checking for python... /nix/store/q7wqmmq18r2m4n4yfb7zkkbpnirc6k91-python3-3.7.1/bin/python
checking for python version... 3.7
checking for python platform... linux
checking for python script directory... ${prefix}/lib/python3.7/site-packages
checking for python extension module directory... ${exec_prefix}/lib/python3.7/site-packages
checking for python-3.7... no
configure: error: Package requirements (python-3.7) were not met:

No package 'python-3.7' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables PYTHON_CFLAGS
and PYTHON_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
builder for '/nix/store/kqwb525yy9y19chcf29mhasayzkgcbkf-btrfs-progs-4.19-armv7l-unknown-linux-gnueabihf.drv' failed with exit code 1
error: build of '/nix/store/kqwb525yy9y19chcf29mhasayzkgcbkf-btrfs-progs-4.19-armv7l-unknown-linux-gnueabihf.drv' failed

$ cat ./btrfs-progs.sh
   #!/bin/sh
   nix-build -I nixpkgs=/home/void/nixpkgs -E 'let t1 = import <nixpkgs> {}; in let pkgs = import <nixpkgs> { crossSystem = t1.l ib.systems.examples.armv7l-hf-multiplatform // { system = "armv7l-linux"; }; }; in pkgs.btrfs-progs '