libmatroska2: use the system zlib if available

It should always be there on Linux/macOS.

libmatroska2/CMakeLists.txt

@@ -6,19 +6,23 @@ option(CONFIG_LZO1X "Enable lzo (de)compression in libmatroska2" ON)
 option(CONFIG_NOCODEC_HELPER "Enable Vorbis frame durations in libmatroska2" ON)
 
 if (CONFIG_ZLIB)
-  include(FetchContent)
+  include(FindZLIB)
 
-  set(ZLIB_BUILD_EXAMPLES OFF)
+  if(NOT ZLIB_FOUND)
+    include(FetchContent)
 
-  FetchContent_Declare(
-      zlib
-      URL      "https://github.com/madler/zlib/releases/download/v1.3.1/zlib-1.3.1.tar.gz"
-      URL_HASH SHA512=580677aad97093829090d4b605ac81c50327e74a6c2de0b85dd2e8525553f3ddde17556ea46f8f007f89e435493c9a20bc997d1ef1c1c2c23274528e3c46b94f
-      DOWNLOAD_EXTRACT_TIMESTAMP TRUE
-  )
+    set(ZLIB_BUILD_EXAMPLES OFF)
 
-  FetchContent_MakeAvailable(zlib)
-  target_compile_definitions("zlib" PRIVATE NO_GZIP)
+    FetchContent_Declare(
+        zlib
+        URL      "https://github.com/madler/zlib/releases/download/v1.3.1/zlib-1.3.1.tar.gz"
+        URL_HASH SHA512=580677aad97093829090d4b605ac81c50327e74a6c2de0b85dd2e8525553f3ddde17556ea46f8f007f89e435493c9a20bc997d1ef1c1c2c23274528e3c46b94f
+        DOWNLOAD_EXTRACT_TIMESTAMP TRUE
+    )
+
+    FetchContent_MakeAvailable(zlib)
+    target_compile_definitions("zlib" PRIVATE NO_GZIP)
+  endif()
 endif()
 
 set(matroska2_SOURCES
@@ -47,7 +51,11 @@ target_include_directories("matroska2"
 target_include_directories("matroska2" PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>)
 
 if (CONFIG_ZLIB)
-  target_link_libraries("matroska2" PRIVATE $<BUILD_INTERFACE:zlib>)
+  if(ZLIB_FOUND)
+    target_link_libraries("matroska2" PRIVATE $<BUILD_INTERFACE:z>)
+  else()
+    target_link_libraries("matroska2" PRIVATE $<BUILD_INTERFACE:zlib>)
+  endif()
 endif(CONFIG_ZLIB)
 if (CONFIG_LZO1X)
   add_subdirectory("minilzo")