Collect platform detection code in a single header

[yallop]

A few parts of the code are preprocessor-conditionalized by platform, e.g. posix_types_stubs.c:

#if (!defined _WIN32 || defined __CYGWIN__) && !defined MINIOS

and ldouble_stubs.c:

#ifdef __ANDROID__

It'd be useful to centralize platform detection in a separate file (ctypes.config.h or similar) and break up the conditional compilation into two steps:

1. ctypes_config.h should define macros that represent whether features are available

   #ifndef __ANDROID__
   #define HAVE_CEXP
   ...

2. the rest of the code should branch on feature availability rather than platform

    #ifdef HAVE_CEXP
     ...

In the longer term we might generate ctypes_config.h via a script run at configuration time.