postinst is in qt4-qmake 4:4.8.1-0ubuntu4.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/sh
set -e
update-alternatives --install \
/usr/bin/qmake qmake "/usr/bin/qmake-qt4" "40" \
--slave /usr/share/man/man1/qmake.1.gz qmake.1.gz \
"/usr/share/man/man1/qmake-qt4.1.gz"
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