append '_cpu' tag correctly

 - fix custom command to work with -B build option

Author: wkpark

CMakeLists.txt

@@ -128,15 +128,15 @@ elseif(BUILD_MPS)
     string(APPEND BNB_OUTPUT_NAME "_mps")
     add_compile_definitions(BUILD_MPS)
     file(MAKE_DIRECTORY "build")
-    add_custom_command(OUTPUT "bitsandbytes/bitsandbytes.metallib"
-                COMMAND xcrun metal -c -o "build/bitsandbytes.air" ${METAL_FILES}
-                COMMAND xcrun metallib "build/bitsandbytes.air" -o "bitsandbytes/bitsandbytes.metallib"
+    add_custom_command(OUTPUT "${CMAKE_SOURCE_DIR}/bitsandbytes/bitsandbytes.metallib"
+                COMMAND xcrun metal -c -o "bitsandbytes.air" "${CMAKE_SOURCE_DIR}/${METAL_FILES}"
+                COMMAND xcrun metallib "bitsandbytes.air" -o "${CMAKE_SOURCE_DIR}/bitsandbytes/bitsandbytes.metallib"
                 DEPENDS "${METAL_FILES}"
                 COMMENT "Compiling Metal kernels"
                 VERBATIM)
-    add_custom_target(metallib DEPENDS "bitsandbytes/bitsandbytes.metallib")
+    add_custom_target(metallib DEPENDS "${CMAKE_SOURCE_DIR}/bitsandbytes/bitsandbytes.metallib")
 else()
-    set(LIBSUFFIX "cpu")
+    string(APPEND BNB_OUTPUT_NAME "_cpu")
     set(GPU_SOURCES)
 endif()