Add FreeBSD support (#358)

* chore: don't use systemd headers with elogind

In file included from src/VTableUtils.c:27:
src/VTableUtils.h:30:10: fatal error: 'systemd/sd-bus.h' file not found
 #include <systemd/sd-bus.h>
          ^~~~~~~~~~~~~~~~~~

* chore: add basu support

Similar to elogind but also supported on non-Linux.

* chore(tests): permit /var/lib/machine-id on non-systemd

https://github.com/elogind/elogind/commit/84fdc0fc61c1
https://git.sr.ht/~emersion/basu/commit/8324e6729231

* chore(ci): add simple freebsd job

Mainly to cover libc++ and basu.

* chore(ci): explicitly pass CMAKE_INSTALL_PREFIX

Some sdbus-cpp tests require configuring system bus. However, Linux
testing relies on writing outside of prefix in order to affect current
system bus instance instead of launching a dedicated one.

* chore(tests): respect CMAKE_INSTALL_PREFIX for system bus config

DBus isn't part of base system on BSDs, so may not use /etc for configs.
Also, testing installation failed as non-root:

$ cmake -DBUILD_TESTS=1 -DCMAKE_INSTALL_PREFIX=/tmp/sdbus-cpp_prefix -DTESTS_INSTALL_PATH=/tmp/sdbus-cpp_prefix/tests
$ cmake --build .
$ cmake --install .
[...]
CMake Error at tests/cmake_install.cmake:105 (file):
  file cannot create directory: /etc/dbus-1/system.d.  Maybe need
  administrative privileges.

* chore(tests): temporarily skip 1 test on FreeBSD to keep CI happy

* chore(ci): run tests in freebsd job

.github/workflows/ci.yml

@@ -48,19 +48,19 @@ jobs:
       run: |
         mkdir build
         cd build
-        cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS="-O0 -g -W -Wextra -Wall -Wnon-virtual-dtor -Werror" -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON ..
+        cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX:PATH=/usr -DCMAKE_CXX_FLAGS="-O0 -g -W -Wextra -Wall -Wnon-virtual-dtor -Werror" -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON ..
     - name: configure-release
       if: matrix.build == 'shared-libsystemd' && matrix.os == 'ubuntu-22.04'
       run: |
         mkdir build
         cd build
-        cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_CXX_FLAGS="-O3 -DNDEBUG -W -Wextra -Wall -Wnon-virtual-dtor -Werror" -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON ..
+        cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_PREFIX:PATH=/usr -DCMAKE_CXX_FLAGS="-O3 -DNDEBUG -W -Wextra -Wall -Wnon-virtual-dtor -Werror" -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON ..
     - name: configure-with-embedded-libsystemd
       if: matrix.build == 'embedded-static-libsystemd'
       run: |
         mkdir build
         cd build
-        cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON -DBUILD_LIBSYSTEMD=ON -DLIBSYSTEMD_VERSION=244 ..
+        cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX:PATH=/usr -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON -DBUILD_CODE_GEN=ON -DBUILD_LIBSYSTEMD=ON -DLIBSYSTEMD_VERSION=244 ..
     - name: make
       run: |
         cd build
@@ -84,3 +84,22 @@ jobs:
           build/sdbus-c++*.deb
           build/sdbus-c++*.ddeb
         retention-days: 10
+  freebsd-build:
+    name: build (freebsd, clang/libc++, basu)
+    runs-on: macos-12 # until https://github.com/actions/runner/issues/385
+    steps:
+    - uses: actions/checkout@v2
+    - name: Test in FreeBSD VM
+      uses: vmactions/freebsd-vm@v0
+      with:
+        copyback: false
+        usesh: true
+        prepare: |
+          pkg install -y cmake ninja pkgconf basu expat googletest
+        run: |
+          cmake -B _build -G Ninja -DBUILD_CODE_GEN=ON -DBUILD_TESTS=ON -DENABLE_PERF_TESTS=ON -DENABLE_STRESS_TESTS=ON
+          cmake --build _build
+          cmake --install _build
+          pkg install -y dbus
+          service dbus onestart
+          ctest --output-on-failure --test-dir _build