wayland: unbreak build without memfd_create

New FreeBSD versions have memfd_create but other BSDs don't.

bar/wayland.c:774:15: error: implicit declaration of function 'memfd_create' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    pool_fd = memfd_create("yambar-wayland-shm-buffer-pool", MFD_CLOEXEC);
              ^
bar/wayland.c:774:62: error: use of undeclared identifier 'MFD_CLOEXEC'
    pool_fd = memfd_create("yambar-wayland-shm-buffer-pool", MFD_CLOEXEC);
                                                             ^

bar/wayland.c

@@ -9,9 +9,7 @@
 #include <pthread.h>
 
 #include <sys/mman.h>
-#ifdef __linux__
-#include <linux/memfd.h>
-#endif
+#include <fcntl.h>
 #include <linux/input-event-codes.h>
 
 #include <pixman.h>
@@ -771,7 +769,16 @@ get_buffer(struct wayland_backend *backend)
     pixman_image_t *pix = NULL;
 
     /* Backing memory for SHM */
+#if defined(MEMFD_CREATE)
     pool_fd = memfd_create("yambar-wayland-shm-buffer-pool", MFD_CLOEXEC);
+#elif defined(__FreeBSD__)
+    // memfd_create on FreeBSD 13 is SHM_ANON without sealing support
+    pool_fd = shm_open(SHM_ANON, O_RDWR | O_CLOEXEC, 0600);
+#else
+    char name[] = "/tmp/yambar-wayland-shm-buffer-pool-XXXXXX";
+    pool_fd = mkostemp(name, O_CLOEXEC);
+    unlink(name);
+#endif
     if (pool_fd == -1) {
         LOG_ERRNO("failed to create SHM backing memory file");
         goto err;

meson.build

@@ -12,6 +12,10 @@ plugs_as_libs = get_option('core-plugins-as-shared-libraries')
 
 cc = meson.get_compiler('c')
 
+if cc.has_function('memfd_create')
+  add_project_arguments('-DMEMFD_CREATE', language: 'c')
+endif
+
 # Compute the relative path used by compiler invocations.
 source_root = meson.current_source_dir().split('/')
 build_root = meson.build_root().split('/')