try fixing FindLZ4 (#1191)

Author: facontidavide

cmake/FindLZ4.cmake

@@ -57,23 +57,26 @@ find_package_handle_standard_args(LZ4
 
 if(LZ4_FOUND)
   message(STATUS "Found LZ4: shared=${LZ4_SHARED_LIBRARY}, static=${LZ4_STATIC_LIBRARY}")
+else()
+  message(WARNING "LZ4 not found")
+  return()
 endif()
 
-mark_as_advanced(LZ4_INCLUDE_DIR LZ4_SHARED_LIBRARY LZ4_STATIC_LIBRARY)
-
 # Create imported targets
-if(NOT TARGET LZ4::lz4_shared)
+if(NOT TARGET LZ4::lz4_shared AND LZ4_SHARED_LIBRARY)
   add_library(LZ4::lz4_shared SHARED IMPORTED GLOBAL)
   set_target_properties(LZ4::lz4_shared PROPERTIES
     IMPORTED_LOCATION           "${LZ4_SHARED_LIBRARY}"
     INTERFACE_INCLUDE_DIRECTORIES "${LZ4_INCLUDE_DIR}"
   )
 endif()
 
-if(NOT TARGET LZ4::lz4_static)
+if(NOT TARGET LZ4::lz4_static AND LZ4_STATIC_LIBRARY)
   add_library(LZ4::lz4_static STATIC IMPORTED GLOBAL)
   set_target_properties(LZ4::lz4_static PROPERTIES
     IMPORTED_LOCATION           "${LZ4_STATIC_LIBRARY}"
     INTERFACE_INCLUDE_DIRECTORIES "${LZ4_INCLUDE_DIR}"
   )
 endif()
+
+mark_as_advanced(LZ4_INCLUDE_DIR LZ4_SHARED_LIBRARY LZ4_STATIC_LIBRARY)

cmake/FindZSTD.cmake

@@ -57,15 +57,15 @@ endif()
 mark_as_advanced(ZSTD_INCLUDE_DIR ZSTD_SHARED_LIBRARY ZSTD_STATIC_LIBRARY)
 
 # Create imported targets
-if(NOT TARGET zstd::libzstd_shared)
+if(NOT TARGET zstd::libzstd_shared AND ZSTD_SHARED_LIBRARY)
   add_library(zstd::libzstd_shared SHARED IMPORTED GLOBAL)
   set_target_properties(zstd::libzstd_shared PROPERTIES
     IMPORTED_LOCATION           "${ZSTD_SHARED_LIBRARY}"
     INTERFACE_INCLUDE_DIRECTORIES "${ZSTD_INCLUDE_DIR}"
   )
 endif()
 
-if(NOT TARGET zstd::libzstd_static)
+if(NOT TARGET zstd::libzstd_static AND ZSTD_STATIC_LIBRARY)
   add_library(zstd::libzstd_static STATIC IMPORTED GLOBAL)
   set_target_properties(zstd::libzstd_static PROPERTIES
     IMPORTED_LOCATION           "${ZSTD_STATIC_LIBRARY}"