Q_OS_LINUX needlessly excludes the BSDs.
has_value() only available in later Boost versions.

Index: src/cpp/desktop/DesktopMain.cpp
--- src/cpp/desktop/DesktopMain.cpp.orig
+++ src/cpp/desktop/DesktopMain.cpp
@@ -52,7 +52,7 @@
 #include <Windows.h>
 #endif
 
-#ifdef Q_OS_LINUX
+#ifdef Q_OS_UNIX
 #include <core/system/PosixSystem.hpp>
 #endif
 
@@ -372,7 +372,7 @@ QString inferDefaultRenderingEngine()
 
 #endif
 
-#ifdef Q_OS_LINUX
+#ifdef Q_OS_UNIX
 
 QString inferDefaultRenderingEngine()
 {
@@ -639,7 +639,7 @@ int main(int argc, char* argv[])
       }
 #endif
 
-#if defined(Q_OS_LINUX) 
+#if defined(Q_OS_UNIX) 
 
       static char noSandbox[] = "--no-sandbox";
 
@@ -1013,7 +1013,7 @@ int main(int argc, char* argv[])
          options.cleanUpScratchTempDir();
 
          boost::optional<SessionServer> pendingReconnect = sessionServers().getPendingSessionServerReconnect();
-         if (pendingReconnect.has_value())
+         if (pendingReconnect)
          {
             // we need to reconnect to the specified session server
             forceSessionServerLaunch = true;