Build failure on x86-linux after #44653

[AenBleidd]

Describe the bug
PR #44653 introduces an issue resulting in build failure on x86-linux

Environment

• OS: [Ubuntu 24.04]
• Compiler: gcc
• Crosscompilation for x86

To Reproduce
Steps to reproduce the behavior:

1. sudo apt-get install -y g++-multilib gcc-multilib
2. cmake -E env CC="gcc -m32" CXX="g++ -m32" CFLAGS="-m32" CXXFLAGS="-m32" LDFLAGS="-m32" cmake -B tests/vcpkg/build -S tests/vcpkg/ -DVCPKG_TARGET_TRIPLET=x86-linux
3. See error

CMake Error at CMakeLists.txt:11 (enable_language):
  The CMAKE_C_COMPILER:

    i686-linux-gnu-gcc

  is not a full path and was not found in the PATH.

  Tell CMake where to find the compiler by setting either the environment
  variable "CC" or the CMake cache entry CMAKE_C_COMPILER to the full path to
  the compiler, or to the compiler name if it is in the PATH.

Expected behavior
Build successful

Failure logs

• Example of the successful build before Define CMAKE_Fortran_COMPILER etc. when cross-compiling #44653: https://github.com/BOINC/boinc/actions/runs/19731804790/job/56534370175
• Example of the failed build after Define CMAKE_Fortran_COMPILER etc. when cross-compiling #44653: https://github.com/BOINC/boinc/actions/runs/19731290033/job/56532818827

Complete test project: https://github.com/BOINC/boinc/tree/master/tests/vcpkg

CC: @stemann, @BillyONeal, @Neumann-A, @Cheney-W

[stemann]

I wonder if the following isn't more correct:

sudo apt-get install -y crossbuild-essential-i386
cmake -B tests/vcpkg/build -S tests/vcpkg/ -DVCPKG_TARGET_TRIPLET=x86-linux

Your installation should at least include the package gcc-i686-linux-gnu.

[AenBleidd]

@stemann, I can definitely test this, but before this was not required to have this package installed

[stemann]

At least, I don't fully understand how gcc-multilib would be enough to get you a cross-compiler (all deps are x86_64 - on x86_64):

$ apt-cache show gcc-multilib
Package: gcc-multilib
...
Depends: cpp (= 4:13.2.0-7ubuntu1), cpp-x86-64-linux-gnu (= 4:13.2.0-7ubuntu1), gcc (= 4:13.2.0-7ubuntu1), gcc-13-multilib (>= 13.2.0-11~), linux-libc-dev

[AenBleidd]

@stemann, ehm... because this is the functionality of gcc? And don't forget, this is a crosscompilation for x86 on x64

[AenBleidd]

@stemann, ok, after a short googling I basically found out that you don't need crossbuild-essential-i386 to build 32-bit binaries on x64 OS, and having g++-multilib gcc-multilib and running compilation with the -m32 key is enough.
So we come to the original topic that that PR broke the build toolchain and need to be fixed.

[AenBleidd]

@stemann, are you able to reproduce this error?