json_pointer.h:suggest minor grammar improvement for pointer doc

Allows the tests to pass when running `make distcheck`

This fixes the 2 broken tests I mentioned at
https://github.com/json-c/json-c/pull/499#discussion_r306998261

build: add option --disable-werror to configure

was typod as -Wwrite-string

to omit -Werror compiler option

Install CMake module files

Installation directories empty with CMake in pkg-config.

CMake was not properly substituting the installation dir variables (they
ended up all empty), so the pkg-config results were also wrongs. For
instance cflags was: -I -I/json-c
Even though json-c was found at configure time, this obviously broke the
build of any application using it.

Convert ChangeLog to valid UTF-8 encoding.

This commit also has the side-effect that errno is set on failed calls
to json_object_set_string(_len).

Instead of using calloc(), call malloc() and initialize the relevant
fields individually.

speedup for 32-bit: ~15%
speedup for 64-bit: ~ 5%

speedup for 32-bit: ~8%
speedup for 64-bit: ~9%

speedup for 32-bit: ~5%
speedup for 64-bit: ~3%

speedup for 32-bit: ~1%
speedup for 64-bit: ~1%

speedup for 32-bit: ~15%
speedup for 64-bit: ~ 2%

This field is set based on o_type when the object is created and it is
not changed during the lifetime of the object. Therefore we can check
o_type to choose the proper delete function in json_object_put(), and
save sizeof(void *) bytes in struct json_object_private.

Fix compiler warnings

Fixed misalignment in JSON string due to space after \n being printed...

json_object_private: save 8 bytes in struct json_object in 64-bit arc…

json_object.c:set errno in json_object_get_double()

json_object.h:document json_object_new_string_len()

Keeping this would be inconsistent with the other documentation

I also added a couple trivial, but related, suggestions.

closes #460

closes #422

README.html:fix link to Doxygen docs, remove WIN32 link

README.md:fix dead "file.html" link

.gitignore:add build directory

STYLE.txt:remove executable permissions

fixes #438
[skip ci]

- there is no need for _ref_count to be uint_fast32_t (the compiler
  might decide to use a 64-bit int). make it uint32_t instead.
- reorder the 32-bit integer fields (o_type and _ref_count) so that
  there is no wasted 4-byte gap after each of them.

Allow use json-c cmake as subproject

Now json-c can be bundled to any cmake-based project and used with
couple lines of code:

add_subdirectory(json-c EXCLUDE_FROM_ALL)
target_link_libraries(MyProject json-c)