SETUPTOOLS_ARG_EXTRA not cleared on 2nd invocation of cmake

[tkruse]

To reproduce, in any new catkin buildspace:

$ cmake ../src
$ cat CMakeCache.txt | grep SETUP
SETUPTOOLS_ARG_EXTRA:STRING=--install-layout=deb
SETUPTOOLS_DEB_LAYOUT:BOOL=ON
$ cmake .. -DSETUPTOOLS_DEB_LAYOUT=OFF
$ cat CMakeCache.txt | grep SETUP
SETUPTOOLS_ARG_EXTRA:STRING=--install-layout=deb
SETUPTOOLS_DEB_LAYOUT:BOOL=OFF

I believe the second time, SETUPTOOLS_ARG_EXTRA should be empty.

This caused these failures on Gentoo systems:
https://code.ros.org/lurker/message/20120426.205157.3adfcbe1.en.html
http://answers.ros.org/question/52021/installing-groovy-from-source-fails

Though I have no idea why distutils in python2.7 on gentoo does not provide a --install-layout option.