Add support for cmake find_package

[MagnusS]

Enables find_package() from other nix-installed packages by adding cmake/includeos-config.cmake.in template, which is then installed to cmake/includeos-config.cmake with the correct paths filled in. This avoids the cmake flags that previously had to be set manually. Updates example.nix to use the new method.

[alfreb]

Right, so this produces the following:

$ cat $(nix-build ~/IncludeOS)/cmake/includeos-config.cmake 
# Config file to be picked up by find_package()

####### Expanded from @PACKAGE_INIT@ by configure_package_config_file() #######
####### Any changes to this file will be overwritten by the next CMake run ####
####### The input file was includeos-config.cmake.in                            ########

get_filename_component(PACKAGE_PREFIX_DIR "${CMAKE_CURRENT_LIST_DIR}/../../" ABSOLUTE)

macro(set_and_check _var _file)
  set(${_var} "${_file}")
  if(NOT EXISTS "${_file}")
    message(FATAL_ERROR "File or directory ${_file} referenced by variable ${_var} does not exist !")
  endif()
endmacro()

macro(check_required_components _NAME)
  foreach(comp ${${_NAME}_FIND_COMPONENTS})
    if(NOT ${_NAME}_${comp}_FOUND)
      if(${_NAME}_FIND_REQUIRED_${comp})
        set(${_NAME}_FOUND FALSE)
      endif()
    endif()
  endforeach()
endmacro()

####################################################################################
set(ARCH "x86_64")
set(INCLUDEOS_PACKAGE /nix/store/3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2-includeos-static-x86_64-unknown-linux-musl-dev)
set(CMAKE_MODULE_PATH /nix/store/3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2-includeos-static-x86_64-unknown-linux-musl-dev/cmake)

So 3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2 is the hash for my IncludeOS package and if I add IncludeOS as a nix dependency it will hook in the path for that to find_package via CMAKE_PREFIX_PATH. We can verify this by going into the nix shell for the example and inspecting its environment:

$ nix-shell example.nix
$ [nix-shell:~/IncludeOS]$ env | grep 3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2
CMAKE_INCLUDE_PATH=... :/nix/store/q3g3nixd69jvrwcz9qd60xmm5f8wq1ig-gtest-static-x86_64-unknown-linux-musl-1.14.0:/nix/store/3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2-includeos-static-x86_64-unknown-linux-musl-dev...
...
CMAKE_PREFIX_PATH=/nix/store/ag0fwkx5sirdlyx0maz55i636rh5lhxm-libcxx-static-x86_64-unknown-linux-musl-18.1.6-dev: ... :/nix/store/3r5nc4b2rirhzqmhmjx0mwjc4v0f3wj2-includeos-static-x86_64-unknown-linux-musl-dev

So we should be able to make a locally built package and instead of exporting INCLUDEOS_PACKAGE pointing to it, we to add it to CMAKE_PREFIX_PATH or install it to a location already known to cmake.

Nice!