If the user changes the .pro file, the Makefile is supposed to be
re-generated by calling qmake again. NMake however lacks a "Makefile
remake feature" like GNU make has.
The generated Makefiles for nmake however have already a proper
Makefile target that can be used to re-generate the Makefile. What was
missing is the dependency from an entry-target in the meta-Makefile.
Now changes in the .pro file trigger a re-generation of
Makefile.Debug/Makefile.Release when calling nmake without target
arguments or with "debug" or "release".
Fixes: QTBUG-29193
Change-Id: I9f2dd5deba4a043ab6c9502bb0b0ba83dc843612
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>