Improve readme to build on linux about osslsigncode

[axi92]

If somebody needs it for a Dockerfile you can use that code as a build stage:

FROM ubuntu:22.04 AS builder

ARG OSSLSIGNCODE_VERSION=2.5

RUN apt update && apt install -y -qq --no-install-recommends build-essential pkg-config libssl-dev libcurl4-openssl-dev clang-15 wget cmake && \
  wget --no-check-certificate https://github.com/mtrojnar/osslsigncode/releases/download/${OSSLSIGNCODE_VERSION}/osslsigncode-${OSSLSIGNCODE_VERSION}.tar.gz && \
  tar -xf osslsigncode-${OSSLSIGNCODE_VERSION}.tar.gz && \
  rm *.tar.gz && \
  cd osslsigncode-${OSSLSIGNCODE_VERSION} && \
  mkdir build && cd build && cmake -S=../ -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=clang-15 && \
  cmake --build . && \
  cp /osslsigncode-${OSSLSIGNCODE_VERSION}/build/osslsigncode /osslsigncode

And copy it later into your image when you need it:
COPY --from=builder /osslsigncode /usr/local/bin/osslsigncode

[mtrojnar]

-DCMAKE_C_COMPILER=clang

Yes, if you force a compiler other than the one installed with build-essential (i.e., gcc), you need to install the compiler first. This is not specific to osslsigncode, and it should not have surprised you.

[axi92]

My bad I don't work often with C or C++.
I took clang because I followed the readme:

osslsigncode/README.md

Line 60 in 44a6768

-DCMAKE_C_COMPILER=clang

[mtrojnar]

Good point! It's indeed not clear that those are examples of configurable build parameters, and not the required build parameters.
@olszomal could you fix it?