allow absolute GNU install dirs

[chayleaf]

CMAKE_INSTALL_LIBDIR is allowed to be an absolute path as per CMake docs. This patch changes the build script to use CMAKE_INSTALL_FULL_LIBDIR instead, and stops using the prefix detection code (many other libraries, such as LLVM, have done so already as it's quite finicky in practice due to absolute dirs being allowed).

If you want to preserve backwards compatibility for setting OATPP_MODULE_LIBDIR, you can take any other approach (like not appending it to install prefix if it's an absolute dir). Absolute dirs are definitely used in the wild, at least on NixOS (where it's necessary due to packages being allowed to have multiple outputs - e.g. one path for headers, one path for libraries, one path for binaries)

[lganzzzo]

Hello @chayleaf ,

Thanks for the PR!
Looks good to me.

However, before merging it we have to make same changes for all other oatpp modules:

• oatpp-consul
• oatpp-curl
• oatpp-libressl
• oatpp-mbedtls
• oatpp-mongo
• oatpp-openssl
• oatpp-postgresql
• oatpp-protobuf
• oatpp-sqlite
• oatpp-ssdp
• oatpp-swagger
• oatpp-websocket
• oatpp-zlib
• oatpp-bob