depends: native_capnp picks system gcc, not CC/CXX #33859

native_capnp seems to ignore the CC/CXX settings. This leads to compile failures on distros that come with an older GCC, such as RHEL-based Linux 8, or the latest OpenSuse Leap:

# ( cd depends && make CC=clang CXX=clang++  -j $(nproc) )

Configuring native_capnp...
-- The CXX compiler identification is GNU 7.5.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ - broken
CMake Error at /usr/share/cmake/Modules/CMakeTestCXXCompiler.cmake:60 (message):
  The C++ compiler

    "/usr/bin/g++"

Native compilers are managed by the build_CC and build_CXX variables, no?

thx, i guess some docs about those settings could help. Otherwise, users will have to read the source code.