BUG: Don't explicitly set the OSX_DEPLOYMENT_TARGET

Setting the OSX_DEPLOYMENT_TARGET implicitly controls which C++
standard library is used libstdc++ vs libc++. Mixing the two in
binaries can result in unstable behavior.

We now implicitly configure the OSX_SYSROOT to the latest SDK, but
require the user to explicitly set the OSX_DEPLOYMENT_TARGET to ensure
compatibility with the build environment.

Co-authored-by: Hans Johnson <hans-johnson@uiowa.edu>
Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Bradley Lowekamp <blowekamp@mail.nih.gov>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23859 3bd1e089-480b-0410-8dfb-8563597acbee