Build mxnet .so lib

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)