Dll creation support for Trans

This patch allows transcc to create dlls.
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.