Preprocessor / conditional compilation

Directives from EC:

#ifdef <symbol>
#ifndef <symbol>
#endif

New directives with ECX:

#else "else"
#elifdef <symbol> "else if defined"
#elifndef <symbol> "else if not defined"

Notes:

Conditional compilation directives are "free-form", and may be put anywhere on a line and does not need to be terminated with newline.

#ifdef DEBUG WriteF('x=\\d\\n', x := y) #else x := y #endif