below commit uses POST_BUILD and PRE_BUILD to generate a fresh copy of
descriptor files for all OTB applications.
b963a177

This has broken dashboard due to a dependency issue.
https://dash.orfeo-toolbox.org/buildSummary.php?buildid=318175

otbQgisDescriptor is not dependent on any otb applications and it is
TRUE. But it has a run-time dependency to all otb applications when we
auto-generating all depscriptors. This situation forces us to add a
dependency so that POST, PRE building targets works. These are only
executed if otbQgisDescriptor target is rebuilt hence, it is not run
everytime with make.

Revert of the initial commit[2] is to get source tree in to a stage which
was reviewed. The first version of post,pre build was pushed directly to
release rather than via MR. Current state in release-6.6 will result in
broken dashboard.

[2] 3f7b6d40

This reverts commit b963a177.

descriptors are generated with a custom target called
'generate_descriptors'. It depends on all OTBAPP.txt
So a missing .txt during installation will try to generate txt file.
This make sure all available applications have a valid descriptor.
There is a internal custom target called clean_descr which depends on
otbQgisDescriptor executable. This target re-create an empty directory
set in OTB_INSTALL_DESCR_DIR under OTB_BINARY_DIR. This make sure every
execution of generate_descriptor target is a clean one.

Finally, custom target is executed by make install using cmake's
install(CODE. Users can either can "make generate_descriptors" manually
or is automatically called during install.