Since you put the Q_OBJECT code in the CPP file and not in a separate header it is not listed in HEADERS and therefore does not get the automated Metaobject compiler (moc) handling from qmake. You need to trigger qmake to generate moc pre-processing in the Makefile, and include the moc generated C++ source, an the easiest way to do that with a one-file-wonder is:
Qt Code: 
  1. #include "main.moc"
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at the bottom of the main.cpp file and re-run qmake.

Normally the header for the QObject class would be in separate file listed in HEADERS. Then qmake ensures that moc is run, the resulting C++ code is compiled, and the result included in your linker run.