TEMPLATE = subdirs
This line says that library.pro is just a container for other projects, contained within the subdirectories listed in the SUBDIRS
variable. Most other variables in library.pro are ignored, except CONFIG += ordered
, which specifies that the subdirectories should be processed in the order in which they are given.
The subprojects which include IInterface2.h all need to have QT += concurrent
in their .pro files.