Need VCPKG_FORCE_SYSTEM_BINARIES env for installing vcpkg dependencies on ARM

[ljishen]

When installing libraries, vcpkg raises this error:

Environment variable VCPKG_FORCE_SYSTEM_BINARIES must be set on arm, s390x, and ppc64le platforms.

Ref: https://github.com/microsoft/vcpkg-tool/blob/2022-02-11/src/vcpkg.cpp#L61

Originally, I was hoping to use the detect_architecture() function in VCEnvironment.cmake to solve this problem, but it turned out detect_architecture cannot detect the ARCH on ARM. Here is a similar issue: microsoft/vcpkg#17832

For cmake < 3.17, CMAKE_HOST_SYSTEM_PROCESSOR uses the command uname -p to get the result but it simply returns "unknown" on ARM: https://cmake.org/cmake/help/v3.16/variable/CMAKE_HOST_SYSTEM_PROCESSOR.html

An unrelated suggestion is that I would like to move the inclusion of Utilities.cmake outside of the file VCEnvironment.cmake, since the functions in Utilities.cmake are no longer exclusively used by that single file.

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[aminya]

@ljishen In #87, I have refactored the detect_architecture function. Is there a special variable that VCPKG uses so I can add it here?

[ljishen]

@aminya No, VCPKG uses cpp for detecting the ARCH: https://github.com/microsoft/vcpkg-tool/blob/2022-02-24/src/vcpkg.cpp#L257-L266

cmake >=3.17.0 fixed this problem by using the result of uname -m on Linux.

And this is what I'm doing in my configuration:

# CMAKE_HOST_SYSTEM_PROCESSOR is not helpful because it outputs "unknown" on
# Linux on ARM.
execute_process(
  COMMAND "uname" "--machine" OUTPUT_VARIABLE MACHINE_HARDWARE_NAME
                                              COMMAND_ERROR_IS_FATAL LAST)
string(TOLOWER "${MACHINE_HARDWARE_NAME}" MACHINE_HARDWARE_NAME_LOWER)

# vcpkg: Environment variable VCPKG_FORCE_SYSTEM_BINARIES must be set on arm,
# s390x, and ppc64le platforms.
# See https://github.com/microsoft/vcpkg-tool/blob/2022-02-24/src/vcpkg.cpp#L257-L266
if((MACHINE_HARDWARE_NAME_LOWER MATCHES "^arm"
    OR MACHINE_HARDWARE_NAME_LOWER MATCHES "^aarch64"
    OR MACHINE_HARDWARE_NAME_LOWER MATCHES "^s390x"
    OR MACHINE_HARDWARE_NAME_LOWER MATCHES "^ppc64"
   )
   AND NOT CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows"
   AND NOT CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
  set(ENV{VCPKG_FORCE_SYSTEM_BINARIES} 1)
endif()

For the placement of including the Utilities.cmake, if eventually we solve the problem of setting the VCPKG_FORCE_SYSTEM_BINARIES for vcpkg running on ARM, I think we should include the Utilities.cmake before including the Vcpkg.cmake in Index.cmake.