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

src/Connection.cpp

@@ -31,7 +31,7 @@
 #include <sdbus-c++/Message.h>
 #include <sdbus-c++/Error.h>
 #include "ScopeGuard.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <unistd.h>
 #include <poll.h>
 #include <sys/eventfd.h>

src/Connection.h

@@ -32,7 +32,7 @@
 #include "IConnection.h"
 #include "ScopeGuard.h"
 #include "ISdBus.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <memory>
 #include <thread>
 #include <string>

src/Error.cpp

@@ -25,7 +25,7 @@
  */
 
 #include <sdbus-c++/Error.h>
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include "ScopeGuard.h"
 
 namespace sdbus

src/Flags.cpp

@@ -25,7 +25,7 @@
  */
 
 #include <sdbus-c++/Flags.h>
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 
 namespace sdbus
 {

src/IConnection.h

@@ -28,7 +28,7 @@
 #define SDBUS_CXX_INTERNAL_ICONNECTION_H_
 
 #include <sdbus-c++/IConnection.h>
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <string>
 #include <memory>
 #include <functional>

src/ISdBus.h

@@ -28,7 +28,7 @@
 #ifndef SDBUS_CXX_ISDBUS_H
 #define SDBUS_CXX_ISDBUS_H
 
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 
 namespace sdbus::internal {
 

src/Message.cpp

@@ -31,7 +31,7 @@
 #include "ISdBus.h"
 #include "IConnection.h"
 #include "ScopeGuard.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <cassert>
 
 namespace sdbus {

src/Object.cpp

@@ -35,7 +35,7 @@
 #include "IConnection.h"
 #include "Utils.h"
 #include "VTableUtils.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <utility>
 #include <cassert>
 

src/Object.h

@@ -29,7 +29,7 @@
 
 #include <sdbus-c++/IObject.h>
 #include "IConnection.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <string>
 #include <map>
 #include <vector>

src/Proxy.cpp

@@ -32,7 +32,7 @@
 #include "sdbus-c++/IConnection.h"
 #include "sdbus-c++/Error.h"
 #include "ScopeGuard.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <cassert>
 #include <chrono>
 #include <utility>

src/Proxy.h

@@ -29,7 +29,7 @@
 
 #include <sdbus-c++/IProxy.h>
 #include "IConnection.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <string>
 #include <memory>
 #include <map>

src/SdBus.cpp

@@ -224,7 +224,12 @@ int SdBus::sd_bus_open_user_with_address(sd_bus **ret, const char* address)
 
 int SdBus::sd_bus_open_system_remote(sd_bus **ret, const char *host)
 {
+#ifdef SDBUS_basu
+    // https://git.sr.ht/~emersion/basu/commit/01d33b244eb6
+    return -EOPNOTSUPP;
+#else
     return ::sd_bus_open_system_remote(ret, host);
+#endif
 }
 
 int SdBus::sd_bus_request_name(sd_bus *bus, const char *name, uint64_t flags)

src/Types.cpp

@@ -27,7 +27,7 @@
 #include <sdbus-c++/Types.h>
 #include <sdbus-c++/Error.h>
 #include "MessageUtils.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <cassert>
 
 namespace sdbus {

src/Utils.h

@@ -28,7 +28,7 @@
 #define SDBUS_CXX_INTERNAL_UTILS_H_
 
 #include <sdbus-c++/Error.h>
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 
 #if LIBSYSTEMD_VERSION>=246
 #define SDBUS_CHECK_OBJECT_PATH(_PATH)                                                                                              \

src/VTableUtils.c

@@ -25,7 +25,7 @@
  */
 
 #include "VTableUtils.h"
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 
 sd_bus_vtable createVTableStartItem(uint64_t flags)
 {

src/VTableUtils.h

@@ -27,7 +27,7 @@
 #ifndef SDBUS_CXX_INTERNAL_VTABLEUTILS_H_
 #define SDBUS_CXX_INTERNAL_VTABLEUTILS_H_
 
-#include <systemd/sd-bus.h>
+#include SDBUS_HEADER
 #include <stdbool.h>
 
 #ifdef __cplusplus