WORK IN PROGRESS
How to build latest QEMU:

#0. Use full Raspbian to make sure all dependencies are met.

1. Copy RPi rootfs to a folder on your PC. You can mount Raspbian image and copy „rootfs” or copy it from your Rpi’s microSD!
Use a command like: sudo cp -a /rootfs/location /destination/dir

2. Install packages on your PC:
sudo apt install systemd-container qemu-user-static

3. Copy qemu executable:
sudo cp /usr/bin/qemu-arm-static /path/to/rootfs/usr/bin

4. Try chrooting using systemd-nspawn:
sudo systemd-nspawn -D /path/to/rootfs

5. You may need this to make networking work. Do something like this:
sudo nano /etc/resolv.conf
Put there this line:
nameserver 8.8.8.8

6. Add src repo:
nano /etc/apt/sources.list
deb-src http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi
(you probably just have to uncomment it)

7. Update the system:
apt update && apt dist-upgrade && apt autoremove && apt autoclean

8. Install QEMU dependencies:
apt install raspberrypi-kernel-headers git libglib2.0-dev libfdt-dev libpixman-1-dev zlib1g-dev git-email libaio-dev libbluetooth-dev libbrlapi-dev libbz2-dev libcap-dev libcap-ng-dev libcurl4-gnutls-dev libgtk-3-dev libibverbs-dev libjpeg8-dev libncurses5-dev librbd-dev librdmacm-dev libsasl2-dev libsdl1.2-dev libseccomp-dev libsnappy-dev libssh2-1-dev libvde-dev libvdeplug-dev libvte-2.90-dev libxen-dev liblzo2-dev valgrind xfslibs-dev libnfs-dev libiscsi-dev bison flex p11-kit libp11-dev libp11-2
//not in Raspbian repo - libnuma-dev
A lot, but this should make sure that we have all dependencies! If you get a message that a package wasn’t found, remove it and run the command again. Run also this:
apt build-dep qemu

Symlink time (trying to solve /usr/bin/ld: cannot find -lp11-kit):
ln -s /usr/lib/arm-linux-gnueabihf/libp11-kit.so.0 /usr/lib/libp11-kit.so.0
ln -s /usr/lib/arm-linux-gnueabihf/libp11-kit.so /usr/lib/libp11-kit.so

8. Don’t compile as root:
exit
sudo systemd-nspawn –user=pi -D /path/to/rootfs
This may result in some QEMU bugs when running sudo (on older versions), but don’t worry – it shouldn’t affect the build.

9. Download QEMU (https://www.qemu.org/download/#source):
git clone https://git.qemu.org/git/qemu.git
cd qemu
git submodule update --init --recursive

10. Configure QEMU:
./configure \
    --prefix=$(cd ..; pwd)/qemu-user-static \
    --static \
    --disable-system \
    --target-list=i386-linux-user
Optional: add
    ----extra-cflags=CFLAGS \
    ----extra-cxxflags=CXXFLAGS
Use these flags to optimize build for your architecture, e.g. --with-arch=armv8-a
11. Compile and „install”
make -jx
x is the number of cores/threads on your pc
make install
cd ../qemu-user-static/bin
for i in *; do cp $i $i-static; done

12.