[PATCH] meson: don't build vstorage where mntent.h is not present

[Nikolay Shirokovskiy <nshirokovskiy@xxxxxxxxxxxxx>]

This should fix CI error:

    ../dist-unpack/libvirt-7.1.0/src/storage/storage_backend_vstorage.c:10:10: fatal error: 'mntent.h' file not found
    #include <mntent.h>
    ^~~~~~~~~~

on freebsd and mac.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@xxxxxxxxxxxxx>
---
 meson.build | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/meson.build b/meson.build
index e3e7ff7..a6b6169 100644
--- a/meson.build
+++ b/meson.build
@@ -1957,8 +1957,20 @@ if conf.has('WITH_LIBVIRTD')
   endif
 
   if not get_option('storage_vstorage').disabled()
-    use_storage = true
-    conf.set('WITH_STORAGE_VSTORAGE', 1)
+    vstorage_enable = true
+
+    if not cc.has_header('mntent.h')
+      if get_option('storage_fs').enabled()
+        error('<mntent.h> is required for the FS storage driver')
+      else
+        vstorage_enable = false
+      endif
+    endif
+
+    if vstorage_enable
+      use_storage = true
+      conf.set('WITH_STORAGE_VSTORAGE', 1)
+    endif
   endif
 
   if not get_option('storage_zfs').disabled()
-- 
1.8.3.1