cmake: Export libcurl and curl targets to use by other cmake projects

[jzakrzewski]

With those exports one should be able to use curl in own project like this (just a stupid example):

cmake_minimum_required(VERSION 2.8.12 FATAL_ERROR)

project(curluser C)

find_package(curl REQUIRED COMPONENTS libcurl OPTIONAL_COMPONENTS curl)

if(curl_curl_FOUND)
    add_custom_target(ex
        COMMAND curl::curl www.example.com -o example.html
    )
endif()

add_executable(user main.c)
target_link_libraries(user curl::libcurl)

[bagder]

The new file is not put into the dist tarball so it fails when built from a release tarball - this is the red travis build:

CMake Error: File /home/travis/build/curl/curl/curl-99.98.97/CMake/curl-config.cmake does not exist.
CMake Error at CMakeLists.txt:1312 (configure_file):
  configure_file Problem configuring file

To fix, add the new file to the CMAKE_DIST variable in the root dir's Makefile.am.

[jzakrzewski]

Ah, snap! I tend to forget about that.

[snikulov]

Why lower letter for prefix curl_ used?
You trying not to clash with FindCURL.cmake module bundled with CMake?

[jzakrzewski]

@snikulov totally by accident (and because the project write its name that way). You think it should be Curl or even CURL?
Frankly speaking this is the first time I'm doing this and all the information about it is quite scattered across various places.
And this is exactly why I'd like some experienced reviewers ;)

[snikulov]

@jzakrzewski Well, If nobody will object, I would like to propose CURL_ prefix for all project's variables and options. What do you think?

[jzakrzewski]

@snikulov So basically use CURL for module name and namespace? Sure, works form me, I'll update it in the evening.
I understand that if someone simply calls find_package(CURL), he'll get it from FindCURL.cmake bundled with the CMake distribution, but find_package(CURL CONFIG) will pick up the config files, right? Hopefully that's not too confusing.

[Lekensteyn]

LGTM

[Lekensteyn]

I would like to propose CURL_ prefix for all project's variables and options. What do you think?

I would recommend "CURL_" for variable names. If you use find_package(Curl) in CMake before 3.2, it would still set the variable CURL_FOUND. Since 3.2, it sets both CURL_FOUND and Curl_FOUND. So to avoid confusion, juse use uppercase.

[jay]

This looks done but one thing, the min ver is bumped from 2.8 to 2.8.8 and I seem to recall there was some issue or reason we went with 2.8 as the minimum version. The commit is 14aa8f0 but it doesn't have a reference url, does anyone remember why 2.8 was chosen as the minimum? Is there some LTS that uses 2.8.0 ? For reference Ubuntu 14 LTS and CentOS 7 have a packaged cmake 2.8.12.2, Ubuntu 10 LTS (past EOL) has 2.8.2 and I'm not sure about Ubuntu 12.

[jzakrzewski]

I have updated the CMake version only as much as it was required for all the bits and pieces to work. Generally we have agreed (see #1010 ) that 2.8.12 is reasonable. However, I have decided not to jump there without any reason. The dumbest reason would be OK though or we make the update right here, right now and get it over with. What do you think?

[Lekensteyn]

Since the autotools build system is still present and cmake is still marked experimental, I think it is OK to bump CMake quite a bit. We can declare CMake on Ubuntu 12.04 unsupported (use autotools in that case).

One good reason to set a higher CMake version is to allow for more freedom, rather than working around missing CMake language features in the old version, you can use the newer version instead. And you also do not have to test with older CMake.

CMake 2.8.12 is OK, I just checked and as of November 2016 RHEL 7 has also been updated with this version. Updated #990 (comment)

[jzakrzewski]

Great, I'll update this PR one more time to bump the version all the way to 2.8.12 and it'll be ready for the next feature window.
I'd like to add the CMake config files to the autotools build also but I'm still not sure how to do it. It would be nice if people could rely on that files (at least from certain version of curl).

[jzakrzewski]

The new commits are intended to be squashed with the first one.
The config file now provides, next to the imported target, the old-style CURL_INCLUDE_DIRS and CURL_LIBRARIES so it should be painless to migrate

[snikulov]

LGTM