[BUG] Build fails with -DMLX_BUILD_PYTHON_BINDINGS=ON

[NAThompson]

Describe the bug

Following the C++ build instructions and defining -DMLX_BUILD_PYTHON_BINDINGS=ON, I get:

CMake Error at python/src/CMakeLists.txt:33 (set_target_properties):
  set_target_properties called with incorrect number of arguments.

To Reproduce

(venv_mlx) ➜  build git:(main) git log
commit 0cea88bcc5e98e81a24d92eed8870a6976999f05 (HEAD -> main, origin/main, origin/HEAD)
Author: Nick <[email protected]>
Date:   Sat Jan 25 16:02:36 2025 -0800

    Use @ matrix multiplication syntax to document matrix-matrix multiplication (#1793)

    Co-authored-by: Nick Thompson <[email protected]>
(venv_mlx) ➜  build git:(main) cmake -DMLX_BUILD_PYTHON_BINDINGS=ON ../
-- Building MLX for arm64 processor on Darwin
-- Building METAL sources
-- Building with macOS SDK version 15.2

-- Accelerate found /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk/System/Library/Frameworks/Accelerate.framework
-- Checking for module 'mpi-c'
--   Package 'mpi-c' not found
-- Could NOT find MPI_C (missing: MPI_C_LIB_NAMES MPI_C_HEADER_DIR MPI_C_WORKS)
-- Checking for module 'mpi-cxx'
--   Package 'mpi-cxx' not found
-- Could NOT find MPI_CXX (missing: MPI_CXX_LIB_NAMES MPI_CXX_HEADER_DIR MPI_CXX_WORKS)
-- Could NOT find MPI (missing: MPI_C_FOUND MPI_CXX_FOUND)
-- Downloading json
-- Using the multi-header code from /Users/nathompson7/mlx/build/_deps/json-src/include/
-- Downloading gguflib
-- Version: 10.2.1
-- Build type:
-- Building Python bindings.
CMake Error at python/src/CMakeLists.txt:33 (set_target_properties):
  set_target_properties called with incorrect number of arguments.


CMake Deprecation Warning at build/_deps/doctest-src/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.


-- Configuring incomplete, errors occurred!

Expected behavior

No error or actionable error message.

Desktop (please complete the following information):

• OS Version: [e.g. MacOS 14.1.2]
• Version [e.g. 0.7.0]

[NAThompson]

I believe this gets us close to the problem:

diff --git a/python/src/CMakeLists.txt b/python/src/CMakeLists.txt
index bcba3a8..d623bfd 100644
--- a/python/src/CMakeLists.txt
+++ b/python/src/CMakeLists.txt
@@ -30,6 +30,10 @@ if(NOT MLX_PYTHON_BINDINGS_OUTPUT_DIRECTORY)
   set(MLX_PYTHON_BINDINGS_OUTPUT_DIRECTORY ${CMAKE_LIBRARY_OUTPUT_DIRECTORY})
 endif()

+if(NOT DEFINED LIBRARY_OUTPUT_DIRECTORY)
+    message(WARNING "LIBRARY_OUTPUT_DIRECTORY is undefined")
+endif()
+
 set_target_properties(
   core
   PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${MLX_PYTHON_BINDINGS_OUTPUT_DIRECTORY}

With this patch applied, we get:

CMake Warning at python/src/CMakeLists.txt:34 (message):
  LIBRARY_OUTPUT_DIRECTORY is undefined


CMake Error at python/src/CMakeLists.txt:37 (set_target_properties):
  set_target_properties called with incorrect number of arguments.