autotools install does not install header files.

```
$ CC=clang-10 CXX=clang++-10 ./configure --prefix=/opt/libccd
$ make
...
$ sudo make install
...
$ ls -R /opt/libccd
/opt/libccd:
lib

/opt/libccd/lib:
libccd.a  libccd.la
```

I expect both the library and the header files to get installed.
