#19 Fix installation of doc files

[Stephan van Veen]

Generation of doc files depends on a CMake option BUILD_DOC and if CMake can find Doxygen and/or LibXslt.
The installation of the doc files however is undonditionally.
With this patch doc files are only installed if peviously generated.

[Daniel Aarno]

Thanks, patch tested and applied in: https://sourceforge.net/p/tclap/code/ci/81b3d2a0c47895c22e9bb8c577f5ab521f76e5d2/