Add workaround for MinGW

New versions of MinGW fail to compile the library with an error stating it ran out of memory. It seems to be related to the heavy template-usage in Eigen: https://bugreports.qt-project.org/browse/QTBUG-38312?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel.

Added workaround by using older versions of MinGW.