Allow OPT to accept optional dashes before flags

man/rgbasm.5

@@ -2482,7 +2482,8 @@ can be used to change some of the options during assembling from within the sour
 takes a comma-separated list of options as its argument:
 .Bd -literal -offset indent
 PUSHO
-    OPT g.oOX, Wdiv       ; acts like command-line -g.oOX -Wdiv
+    OPT g.oOX, Wdiv       ; acts like command-line `-g.oOX -Wdiv`
+    OPT -Wdiv             ; dashes before the options are optional
     DW `..ooOOXX          ; uses the graphics constant characters from OPT g
     PRINTLN $80000000/-1  ; prints a warning about division
 POPO

src/asm/opt.cpp

@@ -53,6 +53,9 @@ void opt_W(char const *flag) {
 }
 
 void opt_Parse(char const *s) {
+	if (s[0] == '-') {
+		++s;
+	}
 	switch (s[0]) {
 	case 'b':
 		if (strlen(&s[1]) == 2) {

test/asm/opt.asm

@@ -1,7 +1,7 @@
 SECTION "test", ROM0
 
 pusho
-	opt p42, Q.4, Wno-div
+	opt p42, -Q.4, Wno-div
 	ds 1
 	println $8000_0000 / -1
 	def n = 3.14
@@ -13,6 +13,6 @@ popo
 	def n = 3.14
 	println "{x:n} = {f:n}"
 
-pusho p99
+pusho -p99
 	ds 1
 popo