qt5_plugins_dir(): use QT_PLUGIN_PATH from environment if set and valid
Adjust libxcb exclusion pattern to actually exclude all of XCB
Merge pull request #139 from lvillani/qt5-plugins-grab-from-plugin-path

qt5_plugins_dir(): use QT_PLUGIN_PATH from environment if set and valid.
Merge pull request #138 from lvillani/adjust-xcb-exclude-regexp

Adjust libxcb exclusion pattern to actually exclude all of XCB.