Dll creation support for Trans

This patch allows transcc to create dlls using the C++ Tool target.
It introduces the new keyword "dllfunction" which triggers Dll creation when used. dllfunctions are excluded from deadcode elimination and a new flag disguises them from functions. dllfunctions have raw names and are therefore not 'munged' this is necessary to satisfy plugin interfaces for third-party applications. when a dll function is used the corresponding file will be a *.dll instead of an exe.  The GC_MODE automatically changes to Mode 2 when a dllfunction is used. There is one remaining quirk: dllfunctions are still exported to the glfw target even if it makes no sense and could break something. I simply don't know how to check for the corresponding target (very likely easy for you?).

You can find  a small code + dll example here:
http://pharmhaus-software.de/public-domain/Dll_example.zip

It would be nice if you could take a look.

Thank you.