diff --git a/doc/buildvariables.md b/doc/buildvariables.md
index 77b6900..78591b5 100644
--- a/doc/buildvariables.md
+++ b/doc/buildvariables.md
@@ -19,6 +19,7 @@
 * `CMAKE_FIND_LIBRARY_SUFFIXES`: sets the library suffixes the build script will
   consider, e.g. set to `.a;.lib` to prefer static Windows libraries or to
   `.dll;.dll.a` to prefer shared Windows libraries
+* variables provided by [GNUInstallDirs](https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html)
 
 ### Custom variables
 The following variables are read by the CMake modules provided by `c++utilities`