Fix sphinx-build out-of-srcdir-build
This patch mostly replaces relative paths with $(srcdir) and $(builddir) as appropriate in doc/Makefile.am since that breaks when you do out-of-srcdir build:
autoreconf -fi
mkdir _build
cd _build
../configure
make distcheck
Also dropped conf_autoconf in favour of -D variable=@value@ in Makefile.am (that's less prone to break).