[release][CMake] Fix GMP test to not always claiming it was found.

Properly quote variable GMP_FOUND.
Do not reuse variable GMP_FOUND.
(cherry picked from commit 317ae3493854d8a37e8b07998887fff1309e6991)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>
[release] Add a man page for dunecontrol

Professional software has man pages.
(cherry picked from commit 25d3fc28cd952932fb27bc70533ab3196523c111)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>