Teach configure to search for multiple commands-with-similar-names. Add NaturalDocs (vs. naturaldocs).

Refactor the doc building rules a bit, sensitize them to the presence or absence of xetex and luatex (for now).

Linux perf tool version 3.2 introduced a new option "--log-fd" defaults
to 0, which leads to error "Failed opening logfd: Illegal argument" when
executing perf tests.

Set logfd to stderr to let perf test work.

Issue #1538

This reverts commit 23e0d16b.

This reverts commit 17d504e0.

Begin shift over to using pandoc, markdown and llnextgen for reference manual. Fix man page URL while at it.

This reverts commit c00ec5f9.

Tarballs for distribution will not have a git repo.

Issue #1274 - should make the build a wee bit faster.

This will keep configure/make from updating submodules, which becomes
very annoying when you are working on LLVM

Configure LLVM correctly, use the right data layout, add the readlink
function back, fix C constants, etc.