Export "selected" CMake target for zstd

[anthonyalayo]

Can we have a consistent CMake library target exported?

For example with the curl project, if the standard CMake global variable BUILD_SHARED_LIBS is set, LIB_SELECTED will match accordingly: https://github.com/curl/curl/blob/master/CMakeLists.txt#L155-L159

# lib flavour selected for example and test programs.
if(BUILD_SHARED_LIBS)
  set(LIB_SELECTED ${LIB_SHARED})
else()
  set(LIB_SELECTED ${LIB_STATIC})
endif()

This is then used as an exported target (in addition to the fixed static/shared targets):

add_library(${LIB_NAME} ALIAS ${LIB_SELECTED})
add_library(${PROJECT_NAME}::${LIB_NAME} ALIAS ${LIB_SELECTED})

This makes pulling in curl quite clean. In comparison, I have to do the following when using fetchcontent with zstd:

set(ZSTD_LIBRARY libzstd_static)
if(BUILD_SHARED_LIBS)
  set(ZSTD_LIBRARY libzstd_shared)
endif()

[Cyan4973]

Does the recently merged #3811 by @teo-tsirpanis answer this issue ?

[teo-tsirpanis]

Curl exposes a unified target even if both static and shared libraries are built.

@Cyan4973 should we do the same here (and in lz4)? And if both are specified, should we prefer static or shared?

[anthonyalayo]

If both are specified, we should rely on whatever BUILD_SHARED_LIBS is set to. This is a global CMake variable than can only be ON or OFF, and configures all subprojects.

[teo-tsirpanis]

The problem is that zstd does not use BUILD_SHARED_LIBS at all. 🤔

[anthonyalayo]

Even if it doesn't, that's what CMake uses, so it should work out well?

[terrelln]

@anthonyalayo, @teo-tsirpanis please let me know if PR #3965 fixes your issues.

[anthonyalayo]

Works, thanks!