Bump PostgreSQL/OpenSSL versions for the binary gem
Add support for cross build for multiple platforms (i386-mingw32 and x64-mingw32).

Update OpenSSL for cross build to 1.0.1e
Update PostgreSQL for cross build to 9.2.3

Remove -lws2_32 for PostgreSQL build. It fails to compile for x64-mingw32
elsewise.
Merge with https://bitbucket.org/ged/ruby-pg
Print proper message in case that cross compilation is not usable.
Mark tests to #socket_io as UNIX-only - they fail on Windows.
Test case 'can wait for NOTIFY events' failed before this fix.
Use #async_exec as alias to #exec on platforms with native threads.

Since we release the GVL for all blocking function calls, now,
#exec has the same behaviour as #async_exec.

Discussed on: https://groups.google.com/d/msg/ruby-pg/OVVO7XjH7Aw/7UIwi6g10PsJ
Add working Connection#socket_io for Windows and remove this method for platforms that don't support it.

This also requires to move the code from Ruby to C.

Use different excudes for :unix and :socket_io.
Avoid compiler warning about unused variable:

../../../../ext/pg_connection.c: In function ‘pgconn_close_socket_io’:
../../../../ext/pg_connection.c:73:6: warning: unused variable ‘ruby_sd’ [-Wunused-variable]
Avoid "no implicit conversion of nil into String" on hosts with no cross-ruby installs
Fix a compiler warning