build: minor tweaks

[Tachi107]

With this patch I've made a few minor changes to CMake configuration:

• CMake config files are now installed to share/ (CMAKE_INSTALL_DATADIR) instead of lib/, as cpp-jwt is header-only, thus arch-independent. See https://github.com/marzer/tomlplusplus/pull/165 for a longer discussion about this.
• Headers are now installed to CMAKE_INSTALL_INCLUDEDIR instead of the hardcoded include/
• I've removed a few redundant options from the install(TARGETS) call, as they are implied by default since CMake 3.14.

[Tachi107]

Thanks for merging! Also, cpp-jwt is now part of Debian :) https://packages.debian.org/unstable/libcpp-jwt-dev