MSVC: ignore empty compilation units (warning LNK4221)

A number of source files have their implementation #ifdef'd out (because
they target another platform).  MSVC warns on empty compilation units
(with warning LNK4221).  Ignore warning 4221 when creating the object
library.

CMakeLists.txt

@@ -164,6 +164,9 @@ IF (MSVC)
 	# /O1 - Optimize for size
 	SET(CMAKE_C_FLAGS_MINSIZEREL "/DNDEBUG /O1 /Oy /GL /Gy ${CRT_FLAG_RELEASE}")
 
+	# /IGNORE:4221 - Ignore empty compilation units
+	SET(CMAKE_STATIC_LINKER_FLAGS "/IGNORE:4221")
+
 	# /DYNAMICBASE - Address space load randomization (ASLR)
 	# /NXCOMPAT - Data execution prevention (DEP)
 	# /LARGEADDRESSAWARE - >2GB user address space on x86