build: stop early if commands are missing

If we fail due to a missing command (always bison)
during the build, it is running many things in parallel
and the error message gets lost in the noise.
Also diagnose bison++.

$ ./make.bash
Your system's bison is bison++, a buggy copy of the original bison.
Go needs the original bison instead.
See http://golang.org/doc/install.html#ctools
$ sudo apt-get remove bison++
... ridiculous amount of output ...
$ ./make.bash
Cannot find 'bison' on search path.
See http://golang.org/doc/install.html#ctools
$ sudo apt-get install bison
... ridiculous amount of output ...
$ ./make.bash
... works

Fixes #1938.
Fixes #1946.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4528137

src/env.bash

@@ -39,13 +39,56 @@ if [ ! -d "$GOBIN" -a "$GOBIN" != "$GOROOT/bin" ]; then
 fi
 
 export OLDPATH=$PATH
-export PATH="$GOBIN":/bin:/usr/bin:$PATH
+export PATH="$GOBIN":$PATH
 
 MAKE=make
 if ! make --version 2>/dev/null | grep 'GNU Make' >/dev/null; then
 	MAKE=gmake
 fi
 
+PROGS="
+	ar
+	awk
+	bash
+	bison
+	chmod
+	cp
+	cut
+	echo
+	ed
+	egrep
+	gcc
+	grep
+	ls
+	mkdir
+	mv
+	pwd
+	rm
+	sed
+	sort
+	tee
+	touch
+	tr
+	true
+	uname
+	uniq
+"
+
+for i in bison ed awk gcc $MAKE; do
+	if ! which $i >/dev/null 2>&1; then
+		echo "Cannot find '$i' on search path." 1>&2
+		echo "See http://golang.org/doc/install.html#ctools" 1>&2
+		exit 1
+	fi
+done
+
+if bison --version 2>&1 | grep 'bison++' >/dev/null 2>&1; then
+	echo "Your system's 'bison' is bison++."
+	echo "Go needs the original bison instead." 1>&2
+	echo "See http://golang.org/doc/install.html#ctools" 1>&2
+	exit 1
+fi
+
 # Tried to use . <($MAKE ...) here, but it cannot set environment
 # variables in the version of bash that ships with OS X.  Amazing.
 eval $($MAKE --no-print-directory -f Make.inc go-env | egrep 'GOARCH|GOOS|GOHOSTARCH|GOHOSTOS|GO_ENV')