Detailed Description

Toolchain File Management

Currently, we have to pair the toolchain files for each architecture with slightly modified "_externals" toolchain file (with that exact postfix). The only difference is that the "_externals" toolchain should not set CMAKE_SYSTEM_PREFIX_PATH or CMAKE_STAGING_PREFIX variables.

The CMAKE_SYSTEM_PREFIX_PATH and CMAKE_STAGING_PREFIX variables have to be explicitly set to the target platform in the default toolchain files to control where things get installed during cross-compilation (i.e. arm64).

Note re: ExternalProject_add Calls

The ABSOLUTE_EXTERNALS_TOOLCHAIN_FILE variable points to the "_externals" toolchain and is supplied as value of the CMAKE_TOOLCHAIN_FILE:FILEPATH variable under the CMAKE_VARS parameter of each ExternalProject_add call; this pattern should be followed for any new ExternalProject_add calls throughout the codebase until a better solution presents itself.

Detailed Rationale.

The installation directories of the external projects are always within the build directory. After they get installed there, the installation results are picked up by other CMake functions and "exported", as needed, for installation with the Physiology code output. Where applicable, this means they will get installed to the CMAKE_STAGING_PREFIX that is specified in the "main" toolchain file.

However, if CMAKE_SYSTEM_PREFIX_PATH and CMAKE_STAGING_PREFIX are set in the toolchain files passed to the CMAKE_TOOLCHAIN_FILE:FILEPATH parameters of ExternalProject_add (i.e. if we pass in the default toolchain files there), they interfere with the <INSTALL_DIR> specified by any other ExternalProject_add arguments, i.e. the location within the build directory where they are actually supposed to end up during the build stage. This is why we need a separate toolchain file for those, until a better solution presents itself.