Append and AppendUnique handle quoting differently

[mwichmann]

Example on a Windows platform and current

env1.Append(CCFLAGS = '/this /is /a /test')
env2.AppendUnique(CCFLAGS = '/this /is /a /test')

giving in one case three arguments and the other a single quoted argument:

cl /Fotestfile.obj /c testfile.cpp /TP /nologo /this /is /a /test
cl /Fotestfile.obj /c testfile.cpp /TP /nologo "/this /is /a /test"

This was split out from #3876 as it is not specific to CPPDEFINES, and that issue was intended to be devoted to CPPDEFINES issues.