CMake/Linux: only link to brotli when using static precompiled libraries

The Brotli library only needs to be explicitly linked when using the
statically linked libraries. When using system libs they're shared, and
the .so loading mechanism takes care of dependencies.

build_files/cmake/platform/platform_unix.cmake

@@ -103,7 +103,9 @@ find_package_wrapper(Zstd REQUIRED)
 
 # FreeType compiled with Brotli compression for woff2.
 find_package_wrapper(Freetype REQUIRED)
-find_package_wrapper(Brotli REQUIRED)
+if(EXISTS ${LIBDIR})
+  find_package_wrapper(Brotli REQUIRED)
+endif()
 
 if(WITH_PYTHON)
   # No way to set py35, remove for now.