<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def build_mxnet(app):
 """Build mxnet .so lib"""

    if not os.path.exists(os.path.join(app.builder.srcdir, '..', 'config.mk')):
        _run_cmd("cd %s/.. && cp make/config.mk config.mk && make -j$(nproc) USE_MKLDNN=0 USE_CPP_PACKAGE=1 " %
                app.builder.srcdir)
    else:
        _run_cmd("cd %s/.. && make -j$(nproc) USE_MKLDNN=0 USE_CPP_PACKAGE=1 " %
                app.builder.srcdir)