main: add fallback mechanism for LLVM commands

On Debian, all LLVM commands have a version suffix (clang-8, ld.lld-8,
wasm-ld-8, etc.). However. Most other distributions only provide a
version prefix for Clang and not for all the other commands.

This commit fixes the issue by trying the command with the version
suffix first and falling back to one without if needed.

main.go

@@ -222,14 +222,11 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 		for i, path := range spec.ExtraFiles {
 			abspath := filepath.Join(root, path)
 			outpath := filepath.Join(dir, "extra-"+strconv.Itoa(i)+"-"+filepath.Base(path)+".o")
-			cmdName := spec.Compiler
-			if name, ok := commands[cmdName]; ok {
-				cmdName = name
+			cmdNames := []string{spec.Compiler}
+			if names, ok := commands[spec.Compiler]; ok {
+				cmdNames = names
 			}
-			cmd := exec.Command(cmdName, append(cflags, "-c", "-o", outpath, abspath)...)
-			cmd.Stdout = os.Stdout
-			cmd.Stderr = os.Stderr
-			err := cmd.Run()
+			err := execCommand(cmdNames, append(cflags, "-c", "-o", outpath, abspath)...)
 			if err != nil {
 				return &commandError{"failed to build", path, err}
 			}
@@ -241,14 +238,11 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 			for _, file := range pkg.CFiles {
 				path := filepath.Join(pkg.Package.Dir, file)
 				outpath := filepath.Join(dir, "pkg"+strconv.Itoa(i)+"-"+file+".o")
-				cmdName := spec.Compiler
-				if name, ok := commands[cmdName]; ok {
-					cmdName = name
+				cmdNames := []string{spec.Compiler}
+				if names, ok := commands[spec.Compiler]; ok {
+					cmdNames = names
 				}
-				cmd := exec.Command(cmdName, append(cflags, "-c", "-o", outpath, path)...)
-				cmd.Stdout = os.Stdout
-				cmd.Stderr = os.Stderr
-				err := cmd.Run()
+				err := execCommand(cmdNames, append(cflags, "-c", "-o", outpath, path)...)
 				if err != nil {
 					return &commandError{"failed to build", path, err}
 				}
@@ -257,11 +251,7 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 		}
 
 		// Link the object files together.
-		if linker, ok := commands[spec.Linker]; ok {
-			err = Link(linker, ldflags...)
-		} else {
-			err = Link(spec.Linker, ldflags...)
-		}
+		err = Link(spec.Linker, ldflags...)
 		if err != nil {
 			return &commandError{"failed to link", executable, err}
 		}