v.cflag: support `#flag $first_existing("/opt/homebrew/lib/libgc.a", "/usr/local/lib/libgc.a")`

vlib/builtin/builtin_d_gcboehm.v

@@ -4,8 +4,8 @@ module builtin
 
 $if static_boehm ? {
 	$if macos {
-		#flag -I/opt/homebrew/include
-		#flag   /opt/homebrew/lib/libgc.a
+		#flag -I$first_existing("/opt/homebrew/include",     "/usr/local/include")
+		#flag   $first_existing("/opt/homebrew/lib/libgc.a", "/usr/local/lib/libgc.a")
 	} $else $if linux {
 		#flag -l:libgc.a
 	} $else {

vlib/v/cflag/cflags.v

@@ -20,11 +20,43 @@ pub fn (c &CFlag) str() string {
 	return 'CFlag{ name: "$c.name" value: "$c.value" mod: "$c.mod" os: "$c.os" cached: "$c.cached" }'
 }
 
+const fexisting_literal = r'$first_existing'
+
+// expand the flag value
+pub fn (cf &CFlag) eval() string {
+	mut value := ''
+	cflag_eval_outer_loop: for i := 0; i < cf.value.len; i++ {
+		x := cf.value[i]
+		if x == `$` {
+			remainder := cf.value[i..]
+			if remainder.starts_with(cflag.fexisting_literal) {
+				sparams := remainder[cflag.fexisting_literal.len + 1..].all_before(')')
+				i += sparams.len + cflag.fexisting_literal.len + 1
+				svalues := sparams.replace(',', '\n').split_into_lines().map(it.trim(' \'"'))
+				mut found_spath := ''
+				for spath in svalues {
+					if os.exists(spath) {
+						found_spath = spath
+						value += spath
+						continue cflag_eval_outer_loop
+					}
+				}
+				panic('>> error: none of the paths $svalues exist')
+				continue
+			}
+		}
+		value += x.ascii_str()
+	}
+	return value
+}
+
 // format flag
 pub fn (cf &CFlag) format() string {
-	mut value := cf.value
+	mut value := ''
 	if cf.cached != '' {
 		value = cf.cached
+	} else {
+		value = cf.eval()
 	}
 	if cf.name in ['-l', '-Wa', '-Wl', '-Wp'] && value.len > 0 {
 		return '$cf.name$value'.trim_space()