Broken out of #30454.
Our QT_STATICPLUGIN macro is effectively equivalent to the Qt’s QT_STATIC macro.
It is easy to see in the _BITCOIN_QT_IS_STATIC macro implementation: https://github.com/bitcoin/bitcoin/blob/ebd82fa9fae13d5a7a395800295dd7fd34185b58/build-aux/m4/bitcoin_qt.m4#L269-L292
No need to handle both macros.