Fix MinGW build using mingw.org's MinGW

Fix MinGW build using mingw.org's MinGW

This allows to build GDB even though the default value of
_WIN32_WINNT is lower than the one needed to expose some
new APIs used here, and leave the test for their actual
support to run time.
* gdb/nat/windows-nat.c (EXTENDED_STARTUPINFO_PRESENT): Define if
not defined.
(create_process_wrapper): Use 'gdb_lpproc_thread_attribute_list'
instead of 'PPROC_THREAD_ATTRIBUTE_LIST' (which might not be defined
at compile time).  This fixes compilation error using mingw.org's
MinGW.