How to install your custom CMake-Find module

I configure and package my library using CMake and CPack. I have written my own find-module: FindMyLib.cmake.

How do I tell CMake/CPack to add this file to the CMake module directory, so that future developers can simply specify FIND_PACKAGE(MyLib) to use my library?

You can set CMAKE_MODULE_PATH and distribute your custom FindFoo.cmake with your project. For example:

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")

The CMake module directory is part of the install tree of CMake itself, and as such you shouldn't be trying to add anything there.

The CMake module directory contains modules which have been written or at least reviewed by Kitware, and adding your own there would give the impression to users of your project that this was the case for your project also.

You'd be better to just install FindMyLib.cmake to one of the places searched by find_package:

<prefix>/                                               (Windows)
<prefix>/(cmake|CMake)/                                 (Windows)
<prefix>/<name>*/                                       (Windows)
<prefix>/<name>*/(cmake|CMake)/                         (Windows)
<prefix>/(lib/<arch>|lib|share)/cmake/<name>*/          (Unix)
<prefix>/(lib/<arch>|lib|share)/<name>*/                (Unix)
<prefix>/(lib/<arch>|lib|share)/<name>*/(cmake|CMake)/  (Unix)
<prefix>/<name>.framework/Resources/                    (Apple)
<prefix>/<name>.framework/Resources/CMake/              (Apple)
<prefix>/<name>.framework/Versions/*/Resources/         (Apple)
<prefix>/<name>.framework/Versions/*/Resources/CMake/   (Apple)
<prefix>/<name>.app/Contents/Resources/                 (Apple)
<prefix>/<name>.app/Contents/Resources/CMake/           (Apple)

See the documentation for find_package for the full details of how find_package searches. Also the CMake packaging tutorial is useful in this case.