[cmake] Always create libzstd target

If both `ZSTD_BUILD_SHARED` and `ZSTD_BUILD_STATIC` are set, then cmake exports the libraries `libzstd_shared` and `libzstd_static` only.
It does not export `libzstd`, which is only exported when exactly one of `ZSTD_BUILD_SHARED` and `ZSTD_BUILD_STATIC` is set.
This PR exports `libzstd` in that case, based on the value of the standard CMake variable [`BUILD_SHARED_LIBS`](https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html).
This ensures that `libzstd` can always be used to refer to the exported zstd library, since the build errors if neither `ZSTD_BUILD_SHARED` nor `ZSTD_BUILD_STATIC` are set.

I tested all the possible combinations of `ZSTD_BUILD_SHARED`, `ZSTD_BUILD_STATIC`, and `BUILD_SHARED_LIBS` and they always worked as expected:
* If only exactly one of `ZSTD_BUILD_SHARED` and `ZSTD_BUILD_STATIC` is set, that is used as `libzstd`.
* Otherwise, libzstd is set based on `BUILD_SHARED_LIBS`.

Fixes #3859.

build/cmake/lib/CMakeLists.txt

@@ -159,6 +159,20 @@ if (ZSTD_BUILD_STATIC AND NOT ZSTD_BUILD_SHARED)
     target_link_libraries(libzstd INTERFACE libzstd_static)
     list(APPEND library_targets libzstd)
 endif ()
+if (ZSTD_BUILD_SHARED AND ZSTD_BUILD_STATIC)
+    # If both ZSTD_BUILD_SHARED and ZSTD_BUILD_STATIC are set, which is the
+    # default, fallback to using BUILD_SHARED_LIBS to determine whether to
+    # set libzstd to static or shared.
+    if (BUILD_SHARED_LIBS)
+        add_library(libzstd INTERFACE)
+        target_link_libraries(libzstd INTERFACE libzstd_shared)
+        list(APPEND library_targets libzstd)
+    else ()
+        add_library(libzstd INTERFACE)
+        target_link_libraries(libzstd INTERFACE libzstd_static)
+        list(APPEND library_targets libzstd)
+    endif ()
+endif ()
 
 # Add specific compile definitions for MSVC project
 if (MSVC)