Define the unified target inside the CMake project, and export it.

This is less error-prone.

build/cmake/lib/CMakeLists.txt

@@ -145,6 +145,16 @@ if (ZSTD_BUILD_STATIC)
     add_definition(libzstd_static ZSTDLIB_STATIC_API)
     add_definition(libzstd_static ZDICTLIB_STATIC_API)
 endif ()
+if (ZSTD_BUILD_SHARED AND NOT ZSTD_BUILD_STATIC)
+    add_library(libzstd INTERFACE)
+    target_link_libraries(libzstd INTERFACE libzstd_shared)
+    list(APPEND library_targets libzstd)
+endif ()
+if (ZSTD_BUILD_STATIC AND NOT ZSTD_BUILD_SHARED)
+    add_library(libzstd INTERFACE)
+    target_link_libraries(libzstd INTERFACE libzstd_static)
+    list(APPEND library_targets libzstd)
+endif ()
 
 # Add specific compile definitions for MSVC project
 if (MSVC)