add posibility to define a symbol for use in conditional compilation

[slyshykO]

somehow send --define:symbol to F# compiler

[kant2002]

Right now use /p to pass additional MSBuild properties. If you can do that with regular MSBuild project, probably you are all set.

[jkone27]

seems also

#if INTERACTIVE 
#else ... 
#endif 

does not work , not sure if related?