Enable Bison warnings

-Wall should be old enough.
Also use %empty instead of comments

src/CMakeLists.txt

@@ -21,7 +21,7 @@ else()
 endif()
 
 find_package(BISON REQUIRED)
-set(BISON_FLAGS "")
+set(BISON_FLAGS "-Wall")
 # Set sompe optimization flags on versions that support them
 if(BISON_VERSION VERSION_GREATER_EQUAL "3.5")
   set(BISON_FLAGS "${BISON_FLAGS} -Dapi.token.raw=true")