Initial work on a C-style preprocessor.

[tangent-vector]

The preprocessor is certainly not 100% complete, but it handles enough to be worth checking in.

Implemented

• Conditional compilation constructs should work:
  • #if including expression parsing and defined(someName)
  • Preprocessor expression can involve macro expansion
  • #ifdef and #ifndef
  • #else and #endif
  • Errors for most bas arrangements
• Basic #include
  • Using a callback interface to actually find/load the files, so it can be consistent with using
• Basic "object-like" #define macros
• #undef
• #warning and #error produce a warning/error respectively, but don't parse the user-defined message
• #line gets syntax checked, but ignored
• #pragma gets ignored

Not implemented:

• #include <foo.h> style. This requires custom work in the lexer.
• "function-like" macros
• Stringification
• Token pasting
• User-defined message for #warning and #error
• GLSL #version directive
• Actually use #line directives. This requires interaction with the lexer.
• Parse #pragma directives and/or leave them for the parser.