fix rock_library with only headers

I tried to define a header-only library using rock_library ...
Which fails miserably since rock_library was calling
add_library unconditionally. This commit makes sure that in the
case where only headers (no source files) are added, add_library
does not get installed. The pkg-config file is still installed,
though.
Merge pull request #4 from rock-core/rock_library_for_header_only_libraries

fix rock_library with only headers