Added some notes regarding ares_build.h

You'll find all c-ares details and news here:

        http://c-ares.haxx.se/

NOTES FOR C-ARES HACKERS

The following notes apply to c-ares version 7.6.1 and later.

* The distributed ares_build.h file is only intended to be used on systems

  which can not run the also distributed configure script.

* The distributed ares_build.h file is generated as a copy of ares_build.h.dist

  when the c-ares source code distribution archive file is originally created.

* If you check out from CVS on a non-configure platform, you must run the

  appropriate buildconf* script to set up ares_build.h and other local files

  before being able of compiling the library.

* On systems capable of running the configure script, the configure process

  will overwrite the distributed ares_build.h file with one that is suitable

  and specific to the library being configured and built, this new file is

  generated from the ares_build.h.in template file.

* If you intend to distribute an already compiled c-ares library you _MUST_

  also distribute along with it the generated ares_build.h which has been

  used to compile it. Otherwise the library will be of no use for the users of

  the library that you have built. It is _your_ responsability to provide this

  file. No one at the c-ares project can know how you have built the library.

* File ares_build.h includes platform and configuration dependant info,

  and must not be modified by anyone. Configure script generates it for you.

* We cannot assume anything else but very basic compiler features being

  present. While libcurl requires an ANSI C compiler to build, some of the

  earlier ANSI compilers clearly can't deal with some preprocessor operators.

* Newlines must remain unix-style for older compilers' sake.

* Comments must be written in the old-style /* unnested C-fashion */