Fix dependency generation on cygwin/mingw32 w clang & gmake. #426

Job	Run time
squeak.cog.spur for win64x64	1m 4s
squeak.cog.spur for win64x64 (DEBUG)	32s
squeak.cog.spur for win64x64 (ASSERT)	1m 3s
squeak.stack.spur for win64x64	2m 11s
squeak.stack.spur for win64x64 (DEBUG)	2m 7s
squeak.stack.spur for win64x64 (ASSERT)	2m 39s
squeak.sista.spur for win64x64	51s
squeak.sista.spur for win64x64 (DEBUG)	1m 5s
squeak.sista.spur for win64x64 (ASSERT)	50s
squeak.cog.spur for win32x86	1m 27s
squeak.cog.spur for win32x86 (DEBUG)	1m 0s
squeak.cog.spur for win32x86 (ASSERT)	1m 17s
squeak.stack.spur for win32x86	2m 37s
squeak.stack.spur for win32x86 (DEBUG)	2m 38s
squeak.stack.spur for win32x86 (ASSERT)	2m 55s
squeak.sista.spur for win32x86	30s
squeak.sista.spur for win32x86 (DEBUG)	1m 20s
squeak.sista.spur for win32x86 (ASSERT)	1m 24s
	27m 30s

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