* Fix CMake install
The install target was trying to install the include directory. It was also
leaving out the sol.hpp header.
The CMake single header and docs generation are also broken, so this commit
disables them by default.
Fixes#585
* Fix CMake single-file header generation and doc generation
* Fix sol2_single target
* Add single-header to ALL
* Install to standard documentation directory
* Make sure single-header artifacts land in the build directory
* More GNUInstallDirs
* pkg-config support
