pgxs patch for win32 (v2)

Dear patchers,

please find attached a very small patch which:
- install win32 headers on "make install"
- install libpostgres.a library under win32 by default (MAKE_DLL=true)
- fix CPPFLAGS under win32 to look for these added header under PGXS

it was tested by Thomas Hallgren to build PL/Java with pgxs.
it may interfere a little bit with Alvaro's patch about
the now useless "remove-all-headers" target in src/include/Makefile

still open question:
- should the MAKE_DLL macro be set by default under cygwin?
- what is the rationnal for this macro? is it still needed?

Have a nice day,

--
Fabien Coelho - coelho(at)cri(dot)ensmp(dot)fr