should warn about identical DESTDIR/TARGET for debug_and_release builds

Even if set to debug, prl files are always in "release mode" when DESTDIR is used.

Example Code:

TEMPLATE = lib 
CONFIG += debug staticlib create_prl 
TARGET = x 
SOURCES += x.cpp 
DESTDIR = out 

int x() { return 1; } 

run these commands:

qmake 
nmake

out/x.prl is in "release mode", for example QMAKE_PRL_LIBS contains release libs (QtCore.lib, etc) when it should be in "debug mode".