What is: The build directory needs to be at the same level as the source directo?

And this is the pro file I have to build my project which is a plugin.

Code:

---

CONFIG      += designer plugin debug_and_release
TARGET      = $$qtLibraryTarget(apcpluginsplugin)
TEMPLATE    = lib
LIBS        += -L. 
....
DESTDIR = $(QTDIR)/plugins/designer
target.path = $$[QT_INSTALL_PLUGINS]/designer
INSTALLS    += target

---

Any idea ? Thanks


Please delete this post (the problem is a contamination of some file of build environment) Solved.