Teach xemu's hardcoded host_os switches about FreeBSD: GL links
against -lGL as on Linux, Vulkan resolves via pkg-config, and the
membarrier probe uses FreeBSD's sys/membarrier.h and SYS_membarrier
(see also patch-util_sys__membarrier.c for the runtime side).

--- meson.build.orig    2026-06-08 04:08:51 UTC
+++ meson.build
@@ -1891,7 +1891,7 @@ if not get_option('opengl').auto() or have_system or h
     opengl_libs=['-framework', 'OpenGL']
   elif host_os == 'windows'
     opengl_libs=['-lopengl32', '-lgdi32']
-  elif host_os == 'linux'
+  elif host_os == 'linux' or host_os == 'freebsd'
     opengl_libs=['-lGL']
   else
     error('Unknown GL platform')
@@ -2389,7 +2389,7 @@ if host_os == 'windows'
 
 if host_os == 'windows'
   vulkan = declare_dependency(compile_args: ['-DVK_USE_PLATFORM_WIN32_KHR'])
-elif host_os == 'linux'
+elif host_os == 'linux' or host_os == 'freebsd'
   vulkan = dependency('vulkan')
 endif
 
@@ -3356,15 +3356,15 @@ elif host_os == 'windows'
   have_membarrier = false
 elif host_os == 'windows'
   have_membarrier = true
-elif host_os == 'linux'
+elif host_os == 'linux' or host_os == 'freebsd'
   have_membarrier = cc.compiles('''
-    #include <linux/membarrier.h>
+    #include <sys/membarrier.h>
     #include <sys/syscall.h>
     #include <unistd.h>
     #include <stdlib.h>
     int main(void) {
-        syscall(__NR_membarrier, MEMBARRIER_CMD_QUERY, 0);
-        syscall(__NR_membarrier, MEMBARRIER_CMD_SHARED, 0);
+        syscall(SYS_membarrier, MEMBARRIER_CMD_QUERY, 0);
+        syscall(SYS_membarrier, MEMBARRIER_CMD_SHARED, 0);
         exit(0);
     }''')
 endif