docs: Add details about plain builds

Mention that build type 'plain' puts the responsibility
for debug features into the hands of the builder.

Related: !2831

diff --git a/docs/reference/gtk/building.md b/docs/reference/gtk/building.md
index 2ee660fb0555d47f232b6f1377f39034f6610d92..51c8802168653b517513eced3b65f7c7b8745b8d 100644 (file)
--- a/docs/reference/gtk/building.md
+++ b/docs/reference/gtk/building.md
@@ -107,7 +107,9 @@ The `plain` build type provided by Meson should only be used when
 packaging GTK, and it's expected that packagers will provide their
 own compiler flags when building GTK. See the previous section for
 the list of environment variables to be used to define compiler and
-linker flags.
+linker flags. Note that with the plain build type, you are also
+responsible for controlling the debugging features of GTK with
+`-DG_ENABLE_DEBUG` and `-DG_DISABLE_CAST_CHECKS`.
 
 ## Dependencies {#dependencies}