Add export targets and config file

[cjh1]

This adds support for building against stempy, either a build tree or install tree.

[cjh1]

find_package(stempy REQUIRED)
target_link_libraries(test PRIVATE stempy::stem Python3::Python)

[swelborn]

I was getting a bunch of strange warnings in my build after running:

cmake -DCMAKE_PREFIX_PATH=/path/to/stempy/build -B build indicating that /home/swelborn/gits/detstream/stempy
was not a file:

cc1: warning: /home/swelborn/gits/detstream/stempy: No such file or directory [-Wmissing-include-dirs]
cc1: warning: /home/swelborn/gits/detstream/stempy: No such file or directory [-Wmissing-include-dirs]

I tried to minimize my configuration to just find a couple packages including stempy:

cmake_minimum_required(VERSION 3.14...3.22)

# ---- Project ----

project(
  detstream
  VERSION 0.1
  LANGUAGES CXX
)

# ---- Required Dependencies ----

find_package(Threads REQUIRED)
find_package(Boost REQUIRED)
find_package(stempy REQUIRED)

get_property(dirs DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY INCLUDE_DIRECTORIES)
foreach(dir ${dirs})
  message(STATUS "dir='${dir}'")
endforeach()

This outputs:

-- dir='/home/swelborn/gits/detstream/stempy'
-- dir='/home/swelborn/gits/stempy/build'

Commenting out find_package(stempy REQUIRED) outputs nothing for the directories function.

Can you reproduce this?

[swelborn]

I think this is where my issue is coming from. ${CMAKE_CURRENT_SOURCE_DIR} is not the same in the CMakeLists that imports. I removed both lines in a test branch here and things still work. I believe the targets should hold this information, right?

[cjh1]

@swelborn I have added an extra target include directory so that we can remove the include_directories(...) otherwise we would break building against an build tree.