Does not build in release mode with cmake 3.10

[t-b]

It complains about unknown add_compile_definitions command (introduced in 3.12). This was added in c19f21a0 (Set release mode flags with add_compile_options, 2020-08-06).

A fix for this issue should also:

• downgrade the installed cmake version in https://github.com/tango-controls/cppTango/blob/tango-9-lts/.travis/debian8/Dockerfile to 3.7
• add a CMAKE_BUILD_TYPE=Release job for travis for debian 8