We have two ways to disable third-party/

  1. make USE_BUNDLED_DEPS=OFF
  2. cmake USE_BUNDLED=OFF

The example used the make option in a cmake context.

The main purpose of this build-type was to avoid unwanted ~/.nvimlog
files (which could get really big, and also affects performance) for
non-devs. But that is no longer necessary since the log system now
avoids non-critical logging by default (#6827).

This essentially reverts 87e5a4131666e44354f280538cbc6bbe52225092

- Establish ERROR log level as "critical". Such errors are rare and will
  be valuable when users encounter unusual circumstances.
- Set -DMIN_LOG_LEVEL=3 for release-type builds

- Remove some unnecessary code: IsHeaderFile is no longer required, as the logic
  to find flags to headers is now built into YCM
- Add function to make paths in flags absolute: It seems YCM is not correctly
  resolving paths in flags to consider `build` as the compiler working
  directory.
- Update documentation.

This reverts commit c13f72ee0a6600963a10e57155d2e814d56599e2.

References #5447.

We use a Makefile which in turn uses cmake. If we wanted to set the install
prefix for cmake, we had to do this so far:

  make CMAKE_FLAGS="-DCMAKE_INSTALL_PREFIX=/tmp/nvim"

That's long and hard to remember. Following the conventions of other Makefiles,
this now works as well and is equivalent:

  make PREFIX=/tmp/nvim

- Check if MIN_LOG_LEVEL value is a number 0-3, default to
  INFO (1) or ignore it in Release mode
- When TRAVIS_CI_BUILD is ON the default is DEBUG (0)
- Add local.mk.example

It hasn't been used in quite a while, so it's probably safe to move it
out of the repository root.