blocks: Fix quadratic behavior in `finalize`

Fixes #57.

For some reason this wasn't getting set in processing
libcmark.pc.in, and we were getting the wrong entry in libcmark.pc.
(See #236)

The new approach sets an internal libdir variable to
lib${LIB_SUFFIX}.  This variable is used both to set the
install destination and in the libcmark.pc.in template.

Closes #236.

However, I'd welcome comments from @juhp who originally
added CMAKE_INSTALL_LIBDIR in #185.  I think that the new
system should work fine with Fedora, since LIB_SUFFIX will
be set appropriately, but some testing is in order.

Due to a mistake, 0.28.1 installed libcmark.a into include/.

Closes #238.

Fixes installation error under some CMake versions, notably kalakris'
CMake backport often used with Travis.

Include GNUInstallDirs in src/CMakeLists.txt

Add sourcepos info for inlines

> 32 nested balanced parens in a link is bananas

Add more pathological tests.

Tests must complete in 8 seconds or are errors.

This allows us to include tests that we don't yet know
how to pass.

* Shift includes around for proper header install

* msvc fixes

Fixes issue #247.

Generate export header for static-only build

* Add baseline test

* Some preliminary work.

* cont'd

* Add footnote reference

* Start postprocessing

* MVP: tests pass

* commonmark footnote out

* Factor out reference/footnote maps

* fix a memory leak & some asserts

* We don't assert/check snprintf elsewhere

* Remove bad linear search, extend test case

* cleanup

* man page update

* add footnotes as option

* bugfix (found in comrak first!)

* Shift static var into renderer struct

…ade sure to free allocated string in commonmark.c

…krefs

tldr, avoid freeing memory before passing it along to another function.

Sometimes, when cleaning up unusued footnotes, two footnote->nodes may
end up referencing each other. As they get free()'d up, this can lead to
problems. Instead, first we unlink every node and _then_ free them up.

Add `aria-label` for accessibility. Add `data-footnotes` as an identifier.

…en class is sanitized

This will describe the footnotes section and improve accessibility. It can be hidden visually.

This reverts commit e4a09ed.

We decided to move this into our own pipeline instead.