CLI11 - a command line parser for C++11 and beyond

CLI11 provides all the features you expect in a powerful command line
parser, with a beautiful, minimal syntax and no dependencies beyond
C++11. It is header only, and comes in a single file form for easy
inclusion in projects. It is easy to use for small projects, but
powerful enough for complex command line projects, and can be
customized for frameworks.

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Starting with v2.6.2, the package installs a pre-built static library as
well as all the headers. In this setup, both header-only and regular
usage of the library are possible:
 - simply including the library headers doesn't require linking with
   anything, so no changes in this scenario
 - looking up the library using pkg-config/pkgconf/CMake defines a macro
   which instructs headers to not define implementation and also sets up
   linking with the static library resulting in faster build times
