Make systemd detection more robust

We don't need to look for this on Windows or macOS, and we need to be
able to gracefully handle systems without pkg-config.

CMakeLists.txt

@@ -302,7 +302,12 @@ if(UNIX AND NOT APPLE)
 endif()
 
 # check for systemd support (socket activation)
-pkg_check_modules(LIBSYSTEMD libsystemd)
+if(UNIX AND NOT APPLE)
+  find_package(PkgConfig)
+  if (PKG_CONFIG_FOUND)
+    pkg_check_modules(LIBSYSTEMD libsystemd)
+  endif()
+endif()
 
 # Generate config.h and make sure the source finds it
 configure_file(config.h.in config.h)