Make the cflags property configurable

So that you can use select statements on it. The current modivation
is to convert a module from make to bp with minimal soong config
variable boilerplate.

Bug: 342006386
Bug: 346922064
Test: m nothing --no-skip-soong-tests
Change-Id: I48d5a11ad3aa65c24cc199458634a5fdbeab2f64
diff --git a/cc/library.go b/cc/library.go
index 090908f..d692b40 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -149,7 +149,7 @@
 
 	Sanitized Sanitized `android:"arch_variant"`
 
-	Cflags []string `android:"arch_variant"`
+	Cflags proptools.Configurable[[]string] `android:"arch_variant"`
 
 	Enabled            *bool    `android:"arch_variant"`
 	Whole_static_libs  []string `android:"arch_variant"`
@@ -464,9 +464,9 @@
 	}
 
 	if library.static() {
-		flags.Local.CFlags = append(flags.Local.CFlags, library.StaticProperties.Static.Cflags...)
+		flags.Local.CFlags = append(flags.Local.CFlags, library.StaticProperties.Static.Cflags.GetOrDefault(ctx, nil)...)
 	} else if library.shared() {
-		flags.Local.CFlags = append(flags.Local.CFlags, library.SharedProperties.Shared.Cflags...)
+		flags.Local.CFlags = append(flags.Local.CFlags, library.SharedProperties.Shared.Cflags.GetOrDefault(ctx, nil)...)
 	}
 
 	if library.shared() {
@@ -2081,8 +2081,8 @@
 
 		// Check libraries in addition to cflags, since libraries may be exporting different
 		// include directories.
-		if len(staticCompiler.StaticProperties.Static.Cflags) == 0 &&
-			len(sharedCompiler.SharedProperties.Shared.Cflags) == 0 &&
+		if len(staticCompiler.StaticProperties.Static.Cflags.GetOrDefault(mctx, nil)) == 0 &&
+			len(sharedCompiler.SharedProperties.Shared.Cflags.GetOrDefault(mctx, nil)) == 0 &&
 			len(staticCompiler.StaticProperties.Static.Whole_static_libs) == 0 &&
 			len(sharedCompiler.SharedProperties.Shared.Whole_static_libs) == 0 &&
 			len(staticCompiler.StaticProperties.Static.Static_libs) == 0 &&