Resolving bug report 892 includes important patches to reduce potential conflict with KDE4 and to make our own *.desktop files XDG compliant (recently Trinity/TDE was added to the list of registered environments: http://standards.freedesktop.org/menu-spec/latest/apb.html).
Resolving this bug report is important to release R14.
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I am fairly certain that what you are looking for is in the file cmake/modules/TDESetupPaths.cmake, specifically the line which sets XDG_APPS_INSTALL_DIR
tde-config is generate from tdecore/tde-config.cpp.cmake, which uses XDG_APPS_INSTALL_DIR as defined during the build process.
Tim
There is a second hardcode for the Autotools build system in admin/acinclude.m4.in, line 535. It looks like this: xdg_appsdir='${datadir}/applications/kde'
Tim