add define support

fmt-c-writer.scm

@@ -256,6 +256,7 @@
     ((or ('struct . _)
          ('union . _)) (walk-struct form))
     (('enum . _) (walk-enum form))
+    (('define name . rest) `(%define ,(atom-to-fmt-c name) ,@rest))
     (else (walk-expr form))))
 
 (define (emit-c sex-forms)

semen.scm

@@ -62,6 +62,7 @@
          ('pub 'var . _)
          ('extern 'var . _)) (process-global-var sex-form acc))
     (('include _) (cons sex-form acc))
+    (('define . _) (cons sex-form acc))
 
     (('import . modules)
      (semen-process-imports (get-public-forms modules) acc))