GNUInstallDirs: Comply with Debian Policy on LIBEXECDIR

If CMAKE_INSTALL_PREFIX is /usr, CMAKE_INSTALL_LIBEXECDIR shall return the same value as CMAKE_INSTALL_LIBDIR on Debian.

While the FHS allows /usr/libexec for internal binaries (https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html#usrlibexec) the Debian Policy Manual states an exception here (section 9.1.1 bullet point 4): https://www.debian.org/doc/debian-policy/ch-opersys#file-system-structure.

Fixes: #21287 (closed)
Backport: release
Topic-rename: GNUInstallDirs-debian