configure: Rename 'extra docs' to 'LaTeX docs'

Issue #111.
configure: Check if pandoc package is available

Issue #111.
build/log: Remove unhelpful comment

Issue #111.
log: Remove README
build/log: Set doc_DATA even if not building LaTeX

Issue #111.
configure: Print whether we're building md docs

Issue #111.
build/log: Add a rule to build md docs

Issue #111.
build/log: Add generated log

Issue #111.
build/log: Remove unneeded pandoc args

Issue #111.
build/log: Fix CLEAN_FILES

Issue #111.
build/log: Add more terse comments
build/log: Compile PDF if not found on make open