feat: 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

tests/integrationtests/DBusStandardInterfacesTests.cpp

@@ -61,12 +61,9 @@ TEST_F(SdbusTestObject, PingsViaPeerInterface)
 
 TEST_F(SdbusTestObject, AnswersMachineUuidViaPeerInterface)
 {
-    // If /etc/machine-id does not exist in your system (which is very likely because you have
-    // a non-systemd Linux), org.freedesktop.DBus.Peer.GetMachineId() will not work. To solve
-    // this, you can create /etc/machine-id yourself as symlink to /var/lib/dbus/machine-id,
-    // and then org.freedesktop.DBus.Peer.GetMachineId() will start to work.
-    if (::access("/etc/machine-id", F_OK) == -1)
-        GTEST_SKIP() << "/etc/machine-id file does not exist, GetMachineId() will not work";
+    if (::access("/etc/machine-id", F_OK) == -1 &&
+        ::access("/var/lib/dbus/machine-id", F_OK) == -1)
+        GTEST_SKIP() << "/etc/machine-id and /var/lib/dbus/machine-id files do not exist, GetMachineId() will not work";
 
     ASSERT_NO_THROW(m_proxy->GetMachineId());
 }