Wiki and branch page build instructions

The build instructions seem alright for Ubuntu distros but I'm missing some details.

1. First, I get the source:
git clone --recurse-submodules https://github.com/ValveSoftware/Proton.git proton

2. At this point, I'm creating the build directory:
cd proton
mkdir build
cd build

3. Here's where I'd like to pass some compiler flags, like CFLAGS=-O3 -march=native
sudo ../configure.sh --enable-ccache --build-name=joelsbuild
(How do I add CFLAGS here? I noticed during the compiling that it passed "-O2 -march=nocona -mtune=core-avx2" which is completely wrong for my system. I have an AMD 5800x)

All of these commands appear to be next and each one seems correct but a I using just one? Or all of them? In what order?
Make install
make dxvk
make vkd3d-proton

Is something like this possible or will it wreck the build?
./configure CFLAGS="-O3 -march=native" CPPFLAGS="-O3 -march=native" CXXFLAGS="-O3 -march=native" X_CFLAGS="-O3 -march=native" OBJCFLAGS="-O3 -march=native" FFLAGS="-O3 -march=native" CCFLAGS="-O3 -march=native" --disable-debug

4. Finally, make install command is unable to find the /root/steam directory. when I do a make --help, I don't see anywhere to add a --prefix for the installation. How do I install this once it's compiled?

echo date '+%s' GIT_DIR=/home/joel/src/proton/proton/.git git describe --tags > /home/joel/src/proton/proton/build/dist/version
if [ ! -d /root/.steam/root ]; then echo >&2 "!! "/root/.steam/root" does not exist, cannot install"; return 1; fi
!! /root/.steam/root does not exist, cannot install
/bin/bash: line 1: return: can only `return' from a function or sourced script
make: *** [../Makefile.in:1141: install] Error 2

Thank you!

Start with the existing Wiki and branch-page build instructions, then inspect configure.sh and the install target around Makefile.in:1141. Verify how compiler flags, the make dxvk and make vkd3d-proton steps, and the Steam installation path are handled. Done means the documented sequence explains these points accurately.