cmd/dist, cmd/go: add environment variable override for pkg-config

Allow overriding default name of `pkg-config` tool via environment
variable PKG_CONFIG (same as used by autoconf pkg.m4 macros). This
facilitates easy cross-compilation of cgo code.

Original patch against Go <= 1.4 was written by
xnox_canonical <dimitri.ledkov@canonical.com> in 2014.
Source: https://codereview.appspot.com/104960043/

Fixes #16253

Change-Id: I31c33ffc3ecbff65da31421e6188d092ab4fe7e4
Reviewed-on: https://go-review.googlesource.com/29991Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cgo/doc.go

@@ -53,6 +53,8 @@ For example:
 	// #include <png.h>
 	import "C"
 
+The default pkg-config tool may be changed by setting the PKG_CONFIG environment variable.
+
 When building, the CGO_CFLAGS, CGO_CPPFLAGS, CGO_CXXFLAGS, CGO_FFLAGS and
 CGO_LDFLAGS environment variables are added to the flags derived from
 these directives.  Package-specific flags should be set using the

src/cmd/dist/build.go

@@ -43,6 +43,7 @@ var (
 	defaultldflags         string
 	defaultcxxtarget       string
 	defaultcctarget        string
+	defaultpkgconfigtarget string
 	rebuildall             bool
 	defaultclang           bool
 
@@ -208,6 +209,12 @@ func xinit() {
 	}
 	defaultcxxtarget = b
 
+	b = os.Getenv("PKG_CONFIG")
+	if b == "" {
+		b = "pkg-config"
+	}
+	defaultpkgconfigtarget = b
+
 	// For tools being invoked but also for os.ExpandEnv.
 	os.Setenv("GO386", go386)
 	os.Setenv("GOARCH", goarch)

src/cmd/dist/buildgo.go

@@ -19,6 +19,7 @@ import (
 //	package main
 //	const defaultCC = <defaultcc>
 //	const defaultCXX = <defaultcxx>
+//	const defaultPkgConfig = <defaultpkgconfig>
 //
 // It is invoked to write cmd/go/zdefaultcc.go
 // but we also write cmd/cgo/zdefaultcc.go
@@ -29,8 +30,9 @@ func mkzdefaultcc(dir, file string) {
 			"package main\n"+
 			"\n"+
 			"const defaultCC = `%s`\n"+
-			"const defaultCXX = `%s`\n",
-		defaultcctarget, defaultcxxtarget)
+			"const defaultCXX = `%s`\n"+
+			"const defaultPkgConfig = `%s`\n",
+		defaultcctarget, defaultcxxtarget, defaultpkgconfigtarget)
 
 	writefile(out, file, writeSkipSame)
 

src/cmd/go/alldocs.go

@@ -1104,6 +1104,8 @@
 // 		Flags that cgo will pass to the compiler when linking.
 // 	CXX
 // 		The command to use to compile C++ code.
+// 	PKG_CONFIG
+// 		Path to pkg-config tool.
 //
 // Architecture-specific environment variables:
 //

src/cmd/go/build.go

@@ -1624,13 +1624,19 @@ func (b *builder) build(a *action) (err error) {
 	return nil
 }
 
+// pkgconfigCmd returns a pkg-config binary name
+// defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
+func (b *builder) pkgconfigCmd() string {
+	return envList("PKG_CONFIG", defaultPkgConfig)[0]
+}
+
 // Calls pkg-config if needed and returns the cflags/ldflags needed to build the package.
 func (b *builder) getPkgConfigFlags(p *Package) (cflags, ldflags []string, err error) {
 	if pkgs := p.CgoPkgConfig; len(pkgs) > 0 {
 		var out []byte
-		out, err = b.runOut(p.Dir, p.ImportPath, nil, "pkg-config", "--cflags", pkgs)
+		out, err = b.runOut(p.Dir, p.ImportPath, nil, b.pkgconfigCmd(), "--cflags", pkgs)
 		if err != nil {
-			b.showOutput(p.Dir, "pkg-config --cflags "+strings.Join(pkgs, " "), string(out))
+			b.showOutput(p.Dir, b.pkgconfigCmd()+" --cflags "+strings.Join(pkgs, " "), string(out))
 			b.print(err.Error() + "\n")
 			err = errPrintedOutput
 			return
@@ -1638,9 +1644,9 @@ func (b *builder) getPkgConfigFlags(p *Package) (cflags, ldflags []string, err e
 		if len(out) > 0 {
 			cflags = strings.Fields(string(out))
 		}
-		out, err = b.runOut(p.Dir, p.ImportPath, nil, "pkg-config", "--libs", pkgs)
+		out, err = b.runOut(p.Dir, p.ImportPath, nil, b.pkgconfigCmd(), "--libs", pkgs)
 		if err != nil {
-			b.showOutput(p.Dir, "pkg-config --libs "+strings.Join(pkgs, " "), string(out))
+			b.showOutput(p.Dir, b.pkgconfigCmd()+" --libs "+strings.Join(pkgs, " "), string(out))
 			b.print(err.Error() + "\n")
 			err = errPrintedOutput
 			return

src/cmd/go/help.go

@@ -465,6 +465,8 @@ Environment variables for use with cgo:
 		Flags that cgo will pass to the compiler when linking.
 	CXX
 		The command to use to compile C++ code.
+	PKG_CONFIG
+		Path to pkg-config tool.
 
 Architecture-specific environment variables:
 

src/make.bash

@@ -47,6 +47,8 @@
 # FC: Command line to run to compile Fortran code for GOARCH.
 # This is used by cgo. Default is "gfortran".
 #
+# PKG_CONFIG: Path to pkg-config tool. Default is "pkg-config".
+#
 # GO_DISTFLAGS: extra flags to provide to "dist bootstrap".
 
 set -e