cgo: add support for CFLAGS in .c files

This patch adds support for passing CFLAGS added in #cgo lines of the
CGo preprocessing phase to the compiler when compiling C files inside
packages. This is expected and convenient but didn't work before.

builder/build.go

@@ -430,7 +430,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		job := &compileJob{
 			description: "compile extra file " + path,
 			run: func(job *compileJob) error {
-				result, err := compileAndCacheCFile(abspath, dir, config)
+				result, err := compileAndCacheCFile(abspath, dir, config.CFlags(), config)
 				job.result = result
 				return err
 			},
@@ -443,12 +443,13 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 	// TODO: do this as part of building the package to be able to link the
 	// bitcode files together.
 	for _, pkg := range lprogram.Sorted() {
+		pkg := pkg
 		for _, filename := range pkg.CFiles {
 			abspath := filepath.Join(pkg.Dir, filename)
 			job := &compileJob{
 				description: "compile CGo file " + abspath,
 				run: func(job *compileJob) error {
-					result, err := compileAndCacheCFile(abspath, dir, config)
+					result, err := compileAndCacheCFile(abspath, dir, pkg.CFlags, config)
 					job.result = result
 					return err
 				},

builder/cc.go

@@ -57,7 +57,7 @@ import (
 //   depfile but without invalidating its name. For this reason, the depfile is
 //   written on each new compilation (even when it seems unnecessary). However, it
 //   could in rare cases lead to a stale file fetched from the cache.
-func compileAndCacheCFile(abspath, tmpdir string, config *compileopts.Config) (string, error) {
+func compileAndCacheCFile(abspath, tmpdir string, cflags []string, config *compileopts.Config) (string, error) {
 	// Hash input file.
 	fileHash, err := hashFile(abspath)
 	if err != nil {
@@ -121,7 +121,7 @@ func compileAndCacheCFile(abspath, tmpdir string, config *compileopts.Config) (s
 		return "", err
 	}
 	depTmpFile.Close()
-	flags := config.CFlags()
+	flags := append([]string{}, cflags...)                                   // copy cflags
 	flags = append(flags, "-MD", "-MV", "-MTdeps", "-MF", depTmpFile.Name()) // autogenerate dependencies
 	flags = append(flags, "-c", "-o", objTmpFile.Name(), abspath)
 	if config.Options.PrintCommands {