Consolidate definition of USE_WIN32API

Consolidate definition of USE_WIN32API

I noticed that USE_WIN32API is defined separately by gdbserver and
gdb.  However, because it is used by code in gdbsupport, it should be
defined by common.m4.  This approach ensures that the code will
continue to work when it is moved to the top level.

gdb/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	* gdbsupport/common.m4 (GDB_AC_COMMON): Define WIN32APILIBS and
	USE_WIN32API when needed.
	* configure.ac (USE_WIN32API): Don't define.
	(WIN32LIBS): Use WIN32APILIBS.
	* configure: Rebuild.

gdb/gdbserver/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	* configure.ac (LIBS): Use WIN32APILIBS.
	(USE_WIN32API): Don't define.
	* configure: Rebuild.

Change-Id: I40d524d5445ebfb452b36f4d0e102f0b1e1089df