https://bitbucket.org/xi/libyaml/issue/10/wrapped-strings-cause-assert-failure

Commenting out the assert makes the scanner do the right thing and
results in just a simple parse failure.

This directory contains instructions and code for getting a libyaml
development environment running.

repro:
  CFLAGS="-Wall -Wextra -Wunused-parameter -Wc++-compat" ./configure && make

which we use for perl, and libyaml is now included in cperl.
Tested with gcc-5 and clang-3.7

There are still a tons of format warnings (%d on 64bit) in
example-deconstructor.c which I skipped.

add 2 new flags:
* problem_nonstrict to yaml_parser_t
* indentless_map to yaml_emitter_t

indentless_map is for backcompat with yaml readers.

It changes the default indent behavior of map elements,
advance the indent the children of the map
according to the spec: 6.1 Indentation Spaces:

      "Each node must be indented further than its parent node.  All sibling nodes
      must use the exact same indentation level.  However the content of each
      sibling node may be further indented independently."
      http://yaml.org/spec/1.2/spec.html#id2777534

This make it readable by YAML.pm (perl),
and behaves the same as all other YAML writers.

problem_nonstrict is to skip aborting on some reader errors:
"control characters are not allowed"
The error is set, but parsing is continued. This allows optionally
accepting invalid string values, copy-as-is, avoiding data-loss.

to be able to set breakpoints.
also fix some pedantic warnings, remove unused code.

optionally allow certian reader errors.
needed for perl YAML validation tests.
See ingydotnet/yaml-libyaml-pm#44

Perl CPAN silently allows any unknown escape chars to pass them to
regexp. Mimic that in nonstrict mode.

we cannot malloc to an anon struct in C++.
typedef yaml_anchors_t

remove unneeded context, which is not set in any macro.

for void context, to fix -Wunused-value warnings

yaml_emitter_write_indicator const char *indicator