Revert "cmake: bump version to 3.7.1"

cmake 3.7 causes serious regressions in some cmake-based packages,
related to how RPATH is handled.

This reverts commit d96fafc3.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>