Fix some issues using MLX in C++ (#739)

* fix preamble build

* fix some issues with using MLX as a dep in C++

mlx/backend/metal/CMakeLists.txt

@@ -4,7 +4,7 @@ add_custom_command(
               ${CMAKE_CURRENT_SOURCE_DIR}/make_compiled_preamble.sh
               ${CMAKE_CURRENT_BINARY_DIR}/compiled_preamble.cpp
               ${CMAKE_C_COMPILER}
-              ${CMAKE_SOURCE_DIR}
+              ${PROJECT_SOURCE_DIR}
     DEPENDS make_compiled_preamble.sh
             kernels/compiled_preamble.h
             kernels/unary.h

mlx/backend/metal/metal.cpp

@@ -1,4 +1,4 @@
-// Copyright © 2023 Apple Inc.
+// Copyright © 2023-2024 Apple Inc.
 
 #include <cstdlib>
 #include <future>
@@ -10,6 +10,10 @@
 
 namespace mlx::core::metal {
 
+bool is_available() {
+  return true;
+}
+
 int max_ops_per_buffer() {
   auto get_val = []() {
     if (const char* buff_str = std::getenv("MLX_MAX_OPS_PER_BUFFER")) {

mlx/backend/metal/metal.h

@@ -1,4 +1,4 @@
-// Copyright © 2023 Apple Inc.
+// Copyright © 2023-2024 Apple Inc.
 
 #pragma once
 
@@ -11,14 +11,7 @@
 
 namespace mlx::core::metal {
 
-constexpr bool is_available() {
-#ifdef _METAL_
-  return true;
-#else
-  return false;
-#endif
-}
-
+bool is_available();
 bool cache_enabled(void);
 void set_cache_enabled(bool enabled);