build: stop early if commands are missing
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 issue 1938.
Fixes issue 1946.

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/4528137

rdp/rogerpack2005-golang,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,scirelli/scirelli-go,d0f/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,mhennings/marcohennings-go,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,scirelli/scirelli-go,scirelli/scirelli-go,npe9/noahevans-go9,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,webfd/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,npe9/noahevans-go9,webfd/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,webfd/go-zh,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,d0f/go-zh,scirelli/scirelli-go,d0f/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,bryanxu/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1