plperl header fix and rationalisation

The perl headers on Win32 try to hijack stdio and lots of other stuff
(like getenv). This is annoying, as it generates lots of warnings, and
probably dangerous as well. To inhibit that we would need to set a flag
before the header inclusion, thus:

#ifdef WIN32
#define WIN32IO_IS_STDIO
#endif

It turns out that we would need to put that in three places (plperl.c,
SPI.xs and spi_internal.h). I propose to rationalise header file use
somewhat, by creating plperl.h, and removing spi_internal.h. plperl.h
will do the above, include all the necessary perl header files, and
define the spi_internal routines.

Comments?

cheers

andrew