Make it easier to override the assembler on Windows.

This is necessary for building with Visual Studio (msbuild)

CMakeLists.txt

@@ -229,15 +229,18 @@ endif()
 
 
 if (MSVC)
+	set(ASSEMBLER ${CMAKE_ASM_COMPILER} CACHE STRING "Assembler to use with Visual Studio (must be gcc / clang!)")
+	message(STATUS "Using custom build commands to work around CMake bugs")
+	message(STATUS "ASM compiler: ${ASSEMBLER}")
 	# CMake is completely broken when you try to build assembly files on Windows.
 	add_custom_command(OUTPUT block_trampolines.obj
-		COMMAND echo ${CMAKE_C_COMPILER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/block_trampolines.S" -o "${CMAKE_BINARY_DIR}/block_trampolines.obj"
-		COMMAND ${CMAKE_C_COMPILER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/block_trampolines.S" -o "${CMAKE_BINARY_DIR}/block_trampolines.obj"
+		COMMAND echo ${ASSEMBLER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/block_trampolines.S" -o "${CMAKE_BINARY_DIR}/block_trampolines.obj"
+		COMMAND ${ASSEMBLER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/block_trampolines.S" -o "${CMAKE_BINARY_DIR}/block_trampolines.obj"
 		MAIN_DEPENDENCY block_trampolines.S
 	)
 	add_custom_command(OUTPUT objc_msgSend.obj
-		COMMAND echo ${CMAKE_C_COMPILER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/objc_msgSend.S" -o "${CMAKE_BINARY_DIR}/objc_msgSend.obj"
-		COMMAND ${CMAKE_C_COMPILER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/objc_msgSend.S" -o "${CMAKE_BINARY_DIR}/objc_msgSend.obj"
+		COMMAND echo ${ASSEMBLER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/objc_msgSend.S" -o "${CMAKE_BINARY_DIR}/objc_msgSend.obj"
+		COMMAND ${ASSEMBLER} ${ASM_TARGET} -c "${CMAKE_SOURCE_DIR}/objc_msgSend.S" -o "${CMAKE_BINARY_DIR}/objc_msgSend.obj"
 		MAIN_DEPENDENCY objc_msgSend.S
 		DEPENDS objc_msgSend.aarch64.S objc_msgSend.arm.S objc_msgSend.mips.S objc_msgSend.x86-32.S objc_msgSend.x86-64.S
 	)