Support CMake Fetch-Content without fighting it.

[acidtonic]

Can we get support for building in Cmake as a Fetch-Content module without all the failures from trying to force external build directories, then failures with git tag parsing? If I clone a detached-shallow for obvious resource savings it fights me and doesn't want to build.

CMake Error at CM_version.cmake:12 (STRING):
STRING sub-command REGEX, mode MATCH needs at least 5 arguments total to
command.
Call Stack (most recent call first):
CMakeLists.txt:26 (INCLUDE)

CMake Error at CMakeLists.txt:28 (PROJECT):
VERSION ".." format invalid.

[stephanecharette]

What do you suggest as a fix? How should this work?

[acidtonic]

Well the "idiomatic" way to build external 3rd party projects with CMake's FetchContent would be the following....

include(FetchContent)
FetchContent_Declare(darknet
  GIT_REPOSITORY  https://github.com/hank-ai/darknet.git
  GIT_TAG         v5.0
  GIT_SHALLOW     TRUE
  GIT_PROGRESS    TRUE
  UPDATE_DISCONNECTED TRUE
)
FetchContent_GetProperties(darknet)
if(NOT darknet_POPULATED)
  FetchContent_Populate(darknet)
  add_subdirectory(${darknet_SOURCE_DIR} ${darknet_BINARY_DIR})
endif()

This strategy builds most projects by checking them out and building with cmake. I use it for things like OpenGL Glad, Apache Paho MQTT, the OAT++ web framework, Intel's OpenCV etc.

This project however is structured such that this is not easily possible. To reproduce simply chuck the above snippet into a skeleton cmake project and it will fail to build. Now swap the git repo with any other normal cmake project and it will pull in that repo and build it.

Thus this is not only my recommended path forward, but also it's test case.

[stephanecharette]

I welcome your PR. I've never built a CMake project that way, so I'm not familiar with FetchContent.