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: Henry Schreiner <henryschreineriii@gmail.com>
Teach the `add_custom_command` and `add_custom_target' commands to
substitute argv0 with the crosscompiling emulator if it is a target with
the `CROSSCOMPILING_EMULATOR` property set.
Prefix test commands with the CROSSCOMPILING_EMULATOR property
for target executables. This allows test suites to be run on the host
when crosscompiling.
If the CMAKE_CROSSCOMPILING_EMULATOR variable is defined, and
CMAKE_CROSSCOMPILING is TRUE, then use CMAKE_CROSSCOMPILING_EMULATOR to run
the try_run executables. This prevents the need to populate
TryRunResults.cmake when cross compiling.
Add CROSSCOMPILING_EMULATOR target property for executables. This is used by
subsequent patches to run exectuables created for the target system when
crosscompiling. The property is initialized by the
CMAKE_CROSSCOMPILING_EMULATOR variable when defined.
Brad King