pgsql: On Windows, set the postmaster executable's stack size to 4MB, so

Log Message:
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On Windows, set the postmaster executable's stack size to 4MB, so that
it agrees with the default value of max_stack_depth.

Modified Files:
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pgsql/src/backend:
Makefile (r1.107 -> r1.108)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/Makefile.diff?r1=1.107&r2=1.108)