v6.15.2 at the time of this writing

• download xmrig: https://github.com/xmrig/xmrig/releases
• copy xmrig/scripts to xmrig-proxy/scripts
• $ cp -frv xmrig/scripts xmrig-proxy/
• $ cd xmrig-proxy && scripts/build_deps.sh
• edit cmake/cmake.flags
• find set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static-libgcc -static-libstdc++") and add -static to it
• result: set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static-libgcc -static-libstdc++ -static")
• $ mkdir -p build && cd build
• $ cmake .. -DXMRIG_DEPS=scripts/deps
• $ make -j`nproc`

xmrig-proxy built this way will work on most systems, although if you want a truly static binary you should compile on Alpine Linux.

Alpine Linux

• apk add git make cmake libstdc++ gcc g++ libuv-dev openssl-dev hwloc-dev
• apk add bash automake libtool m4 autom4te autoconf linux-headers
• (optional) edit build.openssl.sh and change the version from 1.1.1m to 1.1.1n for added security
• scripts/build_deps.sh
• $ mkdir -p build && cd build
• $ cmake .. -DXMRIG_DEPS=scripts/deps
• $ make -j`nproc`