Makefile: make MLX_SO overridable (#111)

Lets downstream callers point at a different MLX artifact (e.g.
libmlx.a for iOS static linking) without editing the Makefile:

    MLX_SO=$(MLX_LIB_DIR)/libmlx.a make

Author: GenericJam

emlx/Makefile

@@ -9,7 +9,7 @@ MLX_SRC_ARCHIVE = $(EMLX_CACHE_DIR)/mlx/$(MLX_VERSION)$(MLX_VARIANT).tar.gz
 MLX_SRC_DIR = $(EMLX_CACHE_DIR)/mlx/src-$(MLX_VERSION)$(MLX_VARIANT)
 MLX_BUILD_DIR = $(EMLX_CACHE_DIR)/mlx/build-$(MLX_VERSION)$(MLX_VARIANT)
 MLX_INSTALL_DIR = $(MLX_DIR)
-MLX_SO = $(MLX_LIB_DIR)/libmlx.dylib
+MLX_SO ?= $(MLX_LIB_DIR)/libmlx.dylib
 
 $(info LIBMLX_ENABLE_DEBUG=$(LIBMLX_ENABLE_DEBUG))
 # Build flags