Bug: http://curl.haxx.se/bug/view.cgi?id=3307835

Currently, Windows cross-compiled autobuilds require inclusion of setup.h
before curl.h to get definitions of CURL_STATICLIB and BUILDING_LIBCURL.

Source files given with relative paths do not have the
-D directory specifier prepended.

Fix compiler warning: conversion may lose significant bits

Fix compiler warning: variable was set but never used

Fix compiler warning: clobber ignored

Also define UNITTESTS macro when building unit test sources.

Fixing compiler warning: external definition with no prior declaration

When switching to HTTP because a HTTP proxy is being used, the existing
handler is now checked if it already is "compatible". This allows the https
handler remain while other non-http handlers will be redirected.

Bug: http://curl.haxx.se/mail/lib-2011-05/0214.html
Reported by: Jerome Robert

Fix compiler warning: variable was set but never used

Fix compiler warning: clobber ignored

Fix compiler warning: variable was set but never used

Fix compiler warning: clobber ignored

Fix compiler warning: external definition with no prior declaration

Fix compiler warning: external definition with no prior declaration

Fix compiler warning: argument is incompatible with corresponding format
string conversion

Enclose header file in an inclusion guard

Fix compiler warning: expression has no effect

Fix compiler warning: `keycheck' might be used uninitialized in this function.
Fix compiler warning: `keybit' might be used uninitialized in this function.

Fix variable declaration placement

Fix missing semicolon

Fix compiler warning: expression has no effect

Fix OOM handling

Fix compiler warning: expression has no effect

Fix compiler warning: unused variable 'data'

Fix compiler warning: enumerated type mixed with another type

Fix compiler warning: enumerated type mixed with another type

Fix compiler warning: enumerated type mixed with another type

Fix compiler warning: enumerated type mixed with another type

Fix compiler warning: enumerated type mixed with another type

Fix compiler warning: empty body in an if-statement

Additionally, prevent multiple inclusions of curl_config.h

winbuild: typo in docs