mingw "make install" should install the .dll file

[kaie]

After running "make install", the destination area doesn't contain librnp-0.dll

Also, it installs some files which probably shouldn't get copied, like cmake files, see below.

Only the following are installed:

-- Install configuration: "Release"
-- Installing: /home/user/local-botan-mingw-32/lib/liblibrnp-0.dll.a
-- Installing: /home/user/local-botan-mingw-32/include/rnp-0/rnp.h
-- Installing: /home/user/local-botan-mingw-32/lib/cmake/rnp/rnp-targets.cmake
-- Installing: /home/user/local-botan-mingw-32/lib/cmake/rnp/rnp-targets-release.cmake
-- Installing: /home/user/local-botan-mingw-32/lib/cmake/rnp/rnp-config.cmake
-- Installing: /home/user/local-botan-mingw-32/lib/cmake/rnp/rnp-config-version.cmake
-- Installing: /home/user/local-botan-mingw-32/lib/pkgconfig/librnp-0.pc
-- Installing: /home/user/local-botan-mingw-32/bin/rnp.exe
-- Installing: /home/user/local-botan-mingw-32/bin/rnpkeys.exe

[kaie]

(Well, I don't know if it makes sense to copy those cmake files. That's just a side comment.)