convert doc/Makefile to CMake

CMake will build the pdf version of the manual if texi2pdf is installed.
It will be installed to DOCDIR/ledger{,3}.pdf, for example
/usr/local/share/ledger/ledger3.pdf.
Also, the man page will be installed to MANDIR/man1/ledger.1

The option BUILD_DOCS is now on by default.

A new option BUILD_WEB_DOCS is used to toggle the generation
of the html version of the manual and the man page (off by default).

All this is added to the 'doc' make target.
Merge pull request #119 from kljohann/cmake.docs

convert doc/Makefile to CMake