CrossCompiling: Load CMAKE_CROSSCOMPILING_EMULATOR from environment
Read CMAKE_CROSSCOMPILING_EMULATOR
from an environment variable of the
same name if not specified with -D
or an initial cache value.
Along with existing environment variable settings such as
CMAKE_TOOLCHAIN_FILE
, cross compilation configuration can be more
completely set via environment variables.
Suggested-by: @henryiii
Edited by Brad King