meson: explicitly disallow unsupported build directory layout

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2512
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3832>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3832>
diff --git a/meson.build b/meson.build
index fb037d7..73ba1b2 100644
--- a/meson.build
+++ b/meson.build
@@ -34,6 +34,10 @@
 
 null_dep = dependency('', required : false)
 
+if get_option('layout') != 'mirror'
+  error('`mirror` is the only build directory layout supported')
+endif
+
 # Arguments for the preprocessor, put these in a separate array from the C and
 # C++ (cpp in meson terminology) arguments since they need to be added to the
 # default arguments for both C and C++.