Add options to CMake build script for adding definitions

[geirhei]

Several options are added in the CMakeLists.txt from CCI used when creating the conan package: https://github.com/conan-io/conan-center-index/pull/14332/files#diff-ea375b62cb31e3394d050d0bb1267949c9abe74b84e714056725e1a28ce4ce3fR35

They should instead be included in the build script for this project so that they are available to others as well. For instance:

option(disable_logs "Disables all logs" OFF)

if (disable_logs)
    add_definitions(-DELPP_DISABLE_LOGS)
endif()

Should also fix #812