[kudu-CR] Note to install "which" command in the installation guide in case of building on RHEL/CentOS.

["Kousuke Saruta (Code Review)" <ge...@cloudera.org>]

Hello Kudu Jenkins,

I'd like you to reexamine a change.  Please visit

    http://gerrit.cloudera.org:8080/5216

to look at the new patch set (#2).

Change subject: Note to install "which" command in the installation guide in case of building on RHEL/CentOS.
......................................................................

Note to install "which" command in the installation guide in case of building on RHEL/CentOS.

I tried to build on Docker container with official CentOS7 image and 
when I tried to build thirdparty libraries, I caught an error message as follows.

+ /root/work/kudu/thirdparty/preflight.py
Running pre-flight checks
-------------------------
Using C compiler: c++
Using C++ compiler: c++

  (Set $CC and $CXX to change compiler)
-------------------------
Checking for autoconf
Traceback (most recent call last):
  File "/root/work/kudu/thirdparty/preflight.py", line 149, in <module>
    sys.exit(main())
  File "/root/work/kudu/thirdparty/preflight.py", line 141, in main
    check_tools()
  File "/root/work/kudu/thirdparty/preflight.py", line 97, in check_tools
    if subprocess.call(["which", tool], stdout=DEV_NULL, stderr=DEV_NULL) != 0:
  File "/usr/lib64/python2.7/subprocess.py", line 524, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib64/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

This message means that "which" command is absent.
Of course, I understand it's very rare case that "which" is absent but I think 
it's better to note just in case to install "which" command in the installation guide.

Change-Id: If02eae6f22a51965120a037cc9a68ddefa418b6c
---
M docs/installation.adoc
1 file changed, 2 insertions(+), 2 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/kudu refs/changes/16/5216/2
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: If02eae6f22a51965120a037cc9a68ddefa418b6c
Gerrit-PatchSet: 2
Gerrit-Project: kudu
Gerrit-Branch: master
Gerrit-Owner: Kousuke Saruta <sa...@oss.nttdata.co.jp>
Gerrit-Reviewer: Kudu Jenkins
Gerrit-Reviewer: Todd Lipcon <to...@apache.org>