acprep: fix verbose build when using ninja
Disable BUILD_DOCS by default

since a separate acprep make doc is necessary to build the documentation
Build info version of documentation
Check for required TeX when building pdf documentation
Improve installation of documentation files

- Install info version of ledger documentation instead of the original
  texinfo sources

- Install html version of ledger documentation when BUILD_WEB_DOCS is ON

- Enable installation of documentation files from --output directory