build: make meson-buildoptions.sh stable

The library directory can change depending on the multilib setup of the host.
It would be even better to detect it in configure with the same algorithm
that Meson uses, but the important thing to avoid confusing developers is
to have identical contents of scripts/meson-buildoptions.sh, independent
of the distro and architecture on which it was created.

So, for now just give a custom default value to libdir.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/meson-buildoptions.py

@@ -61,7 +61,10 @@
 
 # Convert the default value of an option to the string used in
 # the help message
-def value_to_help(value):
+def get_help(opt):
+    if opt["name"] == "libdir":
+        return 'system default'
+    value = opt["value"]
     if isinstance(value, list):
         return ",".join(value)
     if isinstance(value, bool):
@@ -88,7 +91,7 @@ def sh_print(line=""):
 def help_line(left, opt, indent, long):
     right = f'{opt["description"]}'
     if long:
-        value = value_to_help(opt["value"])
+        value = get_help(opt)
         if value != "auto" and value != "":
             right += f" [{value}]"
     if "choices" in opt and long:

scripts/meson-buildoptions.sh

@@ -49,7 +49,7 @@ meson_options_help() {
   printf "%s\n" '  --includedir=VALUE       Header file directory [include]'
   printf "%s\n" '  --interp-prefix=VALUE    where to find shared libraries etc., use %M for'
   printf "%s\n" '                           cpu name [/usr/gnemul/qemu-%M]'
-  printf "%s\n" '  --libdir=VALUE           Library directory [lib64]'
+  printf "%s\n" '  --libdir=VALUE           Library directory [system default]'
   printf "%s\n" '  --libexecdir=VALUE       Library executable directory [libexec]'
   printf "%s\n" '  --localedir=VALUE        Locale data directory [share/locale]'
   printf "%s\n" '  --localstatedir=VALUE    Localstate data directory [/var/local]'