support/dependencies: require at least CMake 3.8

gerbera package in version 1.3 unfortunately now requires CMake >= 3.8
for C++17 macros:
https://github.com/gerbera/gerbera/commit/b5fd39f30fb10385ec228b71377685206a609c3a

So we need to bump our requirement from 3.1 to 3.8. If the host doesn't
have a CMake >= 3.8, Buildroot will build its own host-cmake package.

Also drop patch that relax cmake requirement on json-for-modern-cpp

Fixes:
 - http://autobuild.buildroot.org/results/6405647b47b132ff5d0d211b92d407322d52d507



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>