Make CMake install options for standalone only

[robiwano]

By having a CMake variable:

unset(CLI11_STANDALONE)
if (${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
    set(CLI11_STANDALONE TRUE)
endif()

it can be used to disable install(...) directives, so CLI11 can be used as a submodule in a larger project.

[henryiii]

Please verify this is now fixed in master through #159. I've used CLI11_INSTALL - it deactivates most of the installs if this is a subproject always, hopefully I left in the right bits to allow the CLI11 target to still work as a dependency of a parent project through the config script.

That is, find_package(NewProject CONFIG) should produce working targets even if NewProject's targets depend on CLI11::CLI11 and CLI11 is not found separately.