Target specific architectures for OS X and iOS.
This variable is used to initialize the
property on each target as it is creaed. See that target property
for additional information.
The value of this variable should be set prior to the first
enable_language() command invocation
because it may influence configuration of the toolchain and flags.
It is intended to be set locally by the user creating a build tree.
This variable is ignored on platforms other than OS X.