cmake: Fix exporting -pthread to pkgconfig

[dg0yt]

Before this change, libcares.pc contained -l-pthread.
Noticed in the vcpkg port.

[bradh352]

I'm assuming that means THREADS_PREFER_PTHREAD_FLAG is being set somewhere, since that's not otherwise the default behavior in cmake.

Fix seems reasonable though.

[dg0yt]

I'm assuming that means THREADS_PREFER_PTHREAD_FLAG is being set somewhere, since that's not otherwise the default behavior in cmake.

It is recommended, and it is there:

c-ares/CMakeLists.txt

Line 495 in 8776bf5

SET (CMAKE_THREAD_PREFER_PTHREAD TRUE)

[bradh352]

odd that docs show THREADS_PREFER_PTHREAD_FLAG as the variable name. https://cmake.org/cmake/help/latest/module/FindThreads.html

[dg0yt]

You are right, it is a different identifier.

The issue came up in the android builds. Maybe it came from the toolchain. THREADS_PREFER_PTHREAD_FLAG is more a toolchain setting than a project setting - users probably want the same configuration for a given target platform.