Documentation generated inconditionally when pandoc is found

[triaxx]

If flac 1.5.0 is built on a system where pandoc is installed (e.g. macOS with /usr/local/bin/pandoc), the files flac.html and metaflac.html are installed. On another system without that tool installed, these files are not installed.

In a context of packaging, the list of installed files depends inconditionally on the environment of the developer if pandoc is not added as a dependency of the package.

My current fix is adding "ac_cv_prog_PANDOC=" to the environment of the configure script to emulate that pandoc has not been found.

A better approach could be an option "--with-pandoc" for the configure script.