目录结构如下，在运行目录下边有plugins目录，plugins目录下边有platforms目录，文件如下。
运行时报错如下。
根据经验，一般是由于库找不到造成的。使用ldd查看可执行程序，发现各个库都能找到。
后来发现把plugins下边的platforms目录，复制到运行目录的根目录，就可以成功。但上图的目录结构，不管如何设定LD_LIBRARY_PATH都不起作用。
后来设定了如下环境变量，成功。
Qt帮助：
All Qt programs automatically support a set of command-line options that allow modifying the way Qt will interact with the windowing system. Some of the options are also accessible via environment variables, which are the preferred form if the application can launch GUI sub-processes or other applications (environment variables will be inherited by child processes). When in doubt, use the environment variables.
The options currently supported are the following:
-platform platformName[:options], specifies the Qt Platform Abstraction (QPA) plugin.
Overrides the QT_QPA_PLATFORM environment variable.
-platformpluginpath path, specifies the path to platform plugins.
Overrides the QT_QPA_PLATFORM_PLUGIN_PATH environment variable.