compiler: support for custom flags

[if custom]fn..{} , #flag custom, $if custom {}

vlib/compiler/parser.v

@@ -155,10 +155,22 @@ fn (v mut V) new_parser_from_file(path string) Parser {
 				println('warning: use "$p" file name instead of "$path"')
 			}
 			path_platform = path_ending
-			path_pcguard = platform_postfix_to_ifdefguard(path_ending)
+			path_pcguard = v.platform_postfix_to_ifdefguard(path_ending)
 			break
 		}
 	}
+	
+	if v.compile_defines.len > 0 {
+		for cdefine in v.compile_defines {
+			custom_path_ending := '_d_${cdefine}.v'
+			if path.ends_with(custom_path_ending){
+				path_platform = custom_path_ending
+				path_pcguard = v.platform_postfix_to_ifdefguard('custom $cdefine')
+				break
+			}
+		}
+	}
+	
 	mut p := v.new_parser(new_scanner_file(path))
 	p = {
 		p |