Support per-module MakeVars

This allows setting per-module make variables earlier in the build,
particularly for prebuilt_build_tool users like LEX/M4/BISON. I moved
filegroup over because it's a simpler common interface, but it doesn't
strictly need it.

With this, the last user of the hardcoded cc.m4Cmd variable is gone.

Test: Inspect out/soong/make_vars-*.mk, out/soong/late-*.mk
Test: treehugger
Change-Id: I195b688131feac0c100c338a0749368aa5d50f4f

android/prebuilt_build_tool.go

@@ -29,6 +29,10 @@ type prebuiltBuildToolProperties struct {
 
 	// Extra files that should trigger rules using this tool to rebuild
 	Deps []string `android:"path,arch_variant"`
+
+	// Create a make variable with the specified name that contains the path to
+	// this prebuilt built tool, relative to the root of the source tree.
+	Export_to_make_var *string
 }
 
 type prebuiltBuildTool struct {
@@ -81,6 +85,12 @@ func (t *prebuiltBuildTool) HostToolPath() OptionalPath {
 	return t.toolPath
 }
 
+func (t *prebuiltBuildTool) MakeVars(ctx MakeVarsModuleContext) {
+	if makeVar := String(t.properties.Export_to_make_var); makeVar != "" {
+		ctx.StrictRaw(makeVar, t.toolPath.String())
+	}
+}
+
 var _ HostToolProvider = &prebuiltBuildTool{}
 
 // prebuilt_build_tool is to declare prebuilts to be used during the build, particularly for use