docs: add note about BUILD_* macros

Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/Documentation/howto-build-sys.txt b/Documentation/howto-build-sys.txt
index 22ae8fc6fdc2a1922e2860041027bcd4326cd652..53566140ebb3cc1df47bcd1b738e118f0823da20 100644 (file)
--- a/Documentation/howto-build-sys.txt
+++ b/Documentation/howto-build-sys.txt
@@ -10,6 +10,9 @@ util-linux build system
        ...
        endif
 
+ - "if BUILD_<NAME>" blocks are never nested within another "if BUILD_<NAME>",
+   all dependencies have to be resolved in configure.ac (see UL_REQUIRES_BUILD())
+
  - all BUILD_<NAME> in configure.am are always based on build_<name> variables,
    for example: