Allow for CMAKE_INSTALL_LIBDIR to be absolute

[jonringer]

To address #10045 and #9809

Another attempt at #9822

Package repositories such as Nixpkgs will pass absolute paths to CMAKE variables.

• Also clarify installation of cmake configuration
  files between the build and install directories.

[16:24:12] jon@jon-desktop /home/jon/projects/protobuf (fix-cmake-install-targets2)
$ tree /nix/store/khhxwdyhnc4fd28wsp4dfq0ixfy8ihjn-protobuf-git/lib/cmake
/nix/store/khhxwdyhnc4fd28wsp4dfq0ixfy8ihjn-protobuf-git/lib/cmake/
└── protobuf
    ├── protobuf-config.cmake
    ├── protobuf-config-version.cmake
    ├── protobuf-module.cmake
    ├── protobuf-options.cmake
    ├── protobuf-targets.cmake
    └── protobuf-targets-release.cmake

1 directory, 6 files

[jonringer]

cc @coryan

[jonringer]

Verified that the docker file correctly installs the files in both the build and install directory

FROM docker.io/library/ubuntu:22.04

# Install development tools
ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get update && \
    apt-get --no-install-recommends install -y apt-transport-https apt-utils \
        automake build-essential ccache cmake ca-certificates curl git \
        gcc g++ libc-ares-dev libc-ares2 libcurl4-openssl-dev libre2-dev \
        libssl-dev m4 make ninja-build pkg-config tar wget zlib1g-dev


# Compile and install Abseil
WORKDIR /var/tmp/build/abseil-cpp
RUN curl -sSL https://github.com/abseil/abseil-cpp/archive/20211102.0.tar.gz | \
    tar -xzf - --strip-components=1 && \
    cmake \
      -DCMAKE_BUILD_TYPE=Release \
      -DBUILD_TESTING=OFF \
      -DBUILD_SHARED_LIBS=yes \
      -DCMAKE_CXX_STANDARD=11 \
      -G Ninja -S . -B.build && \
    cmake --build .build && \
    cmake --build .build --target install && \
    ldconfig

# Compile and install Protobuf
WORKDIR /var/tmp/build/protobuf-testonly-z4OsGXYoPj
RUN curl -sSL https://github.com/jonringer/protobuf/archive/fix-cmake-install-targets2.tar.gz | \
    tar -xzf - --strip-components=1 && \
    cmake \
        -DCMAKE_BUILD_TYPE=Release \
        -DBUILD_SHARED_LIBS=yes \
        -Dprotobuf_BUILD_TESTS=OFF \
        -Dprotobuf_ABSL_PROVIDER=package \
        -G Ninja -S . -B.build && \
    cmake --build .build && \
    cmake --build .build --target install  && \
    ldconfig

RUN if find /usr/local/lib/cmake/protobuf -type f xargs grep testonly-z4OsGXYoPj; then false; fi

Result:

$ docker build --tag protobuf -f test.Dockerfile tmp
$ docker run -t -i protobuf /bin/bash

root@af9bc88334da:/var/tmp/build/protobuf-testonly-z4OsGXYoPj# ls /usr/local/lib/cmake/protobuf/
protobuf-config-version.cmake  protobuf-module.cmake   protobuf-targets-release.cmake
protobuf-config.cmake          protobuf-options.cmake  protobuf-targets.cmake

root@af9bc88334da:/var/tmp/build/protobuf-testonly-z4OsGXYoPj# ls /var/tmp/build/protobuf-testonly-z4OsGXYoPj/.build/cmake/p
rotobuf/
protobuf-config-version.cmake  protobuf-config.cmake  protobuf-module.cmake  protobuf-options.cmake  protobuf-targets.cmake

[acozzette]

@jonringer Thanks and sorry for the delay in getting to this!