CMake improvements

[Tachi107]

Hi! First of all, great work! In my opinion, md4c is the most interesting Markdown library of all :)

With this patch set I've reworked a bit the CMake files, to make them a bit more "modern" (what "modern" really means is up to the reader). I've also fixed some actual (small) issues.

I've split this in two separate commits since they are two separate kind of fixes, but if you prefer to have a single commit instead let me know! GitHub's Squash feature kinda sucks and tends to drop commit messages.

Here's the description of the individual changes:

build: CMake improvements

This patch improves and modernizes the CMake build scripts in multiple
ways, while also fixing some bugs:

• Requirements like include directories, compile definitions, and
  libraries are now assigned to targets explicitly. This has the
  benefits of propagating requirements transitively, which is especially
  helpful when using md4c as a subproject or via the CMake Config files.
• The new project() syntax is used, meaning that manually splitting the
  version number in different MD_VERSION_* variables is no longer
  needed. I also made use of the HOMEPAGE_URL option, which requires
  CMake 3.12. This can be reverted in case compatibility with older
  CMake versions is desired.
• Changed ifs to use variables instead of explicitly evaluating them
  with ${} syntax.
• Dropped the MD4C_USE_UTF8 compile definition from CMake code, so
  that users can more easily override the default. UTF-8 is used by
  default automatically if no relevant definition is found since commit
  64bf660.
• Changed DEBUG to only be defined when compiling md4c in debug mode
  via a generator expression (documented in
  https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#build-configurations).
• Different include directory paths are when building and when
  installing the md4c and md4c-html targets, thanks to a generator
  expression (documented in
  https://cmake.org/cmake/help/latest/command/target_include_directories.html).
  This makes the INCLUDES DESTINATION install() option redundant,
  and has been removed.

build: handle absolute include and lib dirs in pc file

With this patch, the pkg-config files are now correctly generated when
the include and/or lib directories are absolute paths outside of
CMAKE_INSTALL_PREFIX, which (if I recall correctly) is commonly the case
on distributions like Nix.

For further information, see
https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path.