COMP: Easier extension build checking environment for midas_package_* vars

When doing extension experimental build, having to systematically passing
the option -DMIDAS_PACKAGE_EMAIL:STRING=<email> and
-DMIDAS_PACKAGE_API_KEY:STRING=<api_key> was tedious.

After this commit, the build system will first check if corresponding
variables are set in the environment.

On unix system, the variables could be set in the ~/.bash_aliases file
using the following syntax:

export MIDAS_PACKAGE_EMAIL=YOUR_EMAIL
export MIDAS_PACKAGE_API_KEY=YOUR_API_KEY

On windows, you could follow these instructions:
http://www.nextofwindows.com/how-to-addedit-environment-variables-in-windows-7/

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22457 3bd1e089-480b-0410-8dfb-8563597acbee