Include the freebsd.pri file provided by the port, to pass FreeBSD specific
settings to gn.

--- mkspecs/features/functions.prf.orig
+++ mkspecs/features/functions.prf
@@ -15,7 +15,7 @@
 
 defineTest(isPlatformSupported) {
   QT_FOR_CONFIG += gui-private
-  linux {
+  unix {
     if(!gcc:!clang)|intel_icc {
       skipBuild("Qt WebEngine on Linux requires clang or GCC.")
       return(false)
@@ -367,6 +367,7 @@
     }
     macos: include($$QTWEBENGINE_ROOT/src/core/config/mac_osx.pri)
     win32: include($$QTWEBENGINE_ROOT/src/core/config/windows.pri)
+    freebsd: include($$QTWEBENGINE_ROOT/src/core/config/freebsd.pri)
     isEmpty(gn_args): error(No gn_args found please make sure you have valid configuration.)
     return($$gn_args)
 }
@@ -386,6 +387,7 @@
     macos: return(mac)
     win32: return(win)
     linux: return(linux)
+    freebsd: return(bsd)
     error(Unsupported platform)
     return(unknown)
 }