BUG: Ensure ExtensionBuildSystem tests propagate CMAKEOSX* vars

This commit fixes error like this one:

7: -- Setting OSX_ARCHITECTURES to 'x86_64' as none was specified.
7: -- Setting OSX_SYSROOT to latest '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk' as none was specified.
7: CMake Error at /Users/inorton/git/slcr/r4/CMake/SlicerBlockSetCMakeOSXVariables.cmake:102 (message):
7:   The OSX_SYSROOT is set to version 10.12 (>10.8) and OSX_DEPLOYMENT_TARGET
7:   is not explicitly set!
7:
7:   Since:
7:
7:    (1) the default runtime associated with >=10.9 deployment target is 'libc++'.[1]
7:    (2) the default runtime associated with <=10.8 deployment target is 'libstdc++'.
7:    (3) Qt support for 'macx-clang-libc++' is listed as 'unsupported' mkspecs.
7:    (4) Qt binaries may be build against 'libstdc++' or 'libc++'.
7:    (5) Mixing the two different runtime in binaries is unstable.
7:     [1]http://stackoverflow.com/questions/19637164/c-linking-error-after-upgrading-to-mac-os-x-10-9-xcode-5-0-1/19637199#19637199
7:
7:   --------------------------------
7:
7:   Run '$otool -L $(which qmake) |grep lib.\*c++' to check what library Qt is
7:   built against:
7:
7:    (1) if it is libstdc++ then add '-DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.8' (or older) to the cmake command line.
7:    (2) if it is libc++ then add '-DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.9' (or newer) to the cmake command line.
7:
7: Call Stack (most recent call first):
7:   CMakeLists.txt:14 (include)
7:

Reported-by: Isaiah Norton <isaiah.norton@gmail.com>
Tested-by: Isaiah Norton <isaiah.norton@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25435 3bd1e089-480b-0410-8dfb-8563597acbee