Install header files?

[smspillaz]

Right now header files aren't installed as part of the build process.

At least for linux packaging, installing the header file is sort of a pre-requisite if code using this library want to be part of a distro's package set, since there's a general skepticism of any sort of static linking. Fortunately, there's a BUILD_SHARED_LIBS option, so to make a -dev package, one really only need to install the header file.

This raises the side question of API and/or ABI stability. Its probably fine to just say "no guarantees for now, all rdepends will need to be rebuilt on upgrade".