Split installation into 3 sub targets

Split installation into 3 sub targets

Namely:

- `install-lib`: library files and headers.
- `install-doc`: man pages.
- `install-pc` : pkg-config file.

Some users don't want to install everything.  Those who pull Monocypher
directly from git, despite strong suggestion to use a tarball instead,
can be especially annoyed at the mandoc dependency.

We could also split the library and includes, but this feels overkill.