chat(build): add conftest for std::optional::transform

[cebtenzzre]

This will prevent users from wasting time waiting for GPT4All to compile if their compiler does not support std::optional::transform, which we require. Success looks like this:

-- Checking for __cpp_lib_optional >= 202110L
-- Checking for __cpp_lib_optional >= 202110L - pass

Failure looks like this:

$ cmake -B build -DCMAKE_C_COMPILER=gcc-11 -DCMAKE_CXX_COMPILER=g++-11
-- The CXX compiler identification is GNU 11.3.0
-- The C compiler identification is GNU 11.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/g++-11 - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc-11 - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Checking for __cpp_lib_optional >= 202110L
-- Checking for __cpp_lib_optional >= 202110L - fail
CMake Error at CMakeLists.txt:46 (message):
  The C++ compiler

    "/usr/bin/g++-11"

  is too old to support __cpp_lib_optional >= 202110L.

  Please specify a newer compiler via
  -DCMAKE_C_COMPILER/-DCMAKE_CXX_COMPILER.
Call Stack (most recent call first):
  CMakeLists.txt:56 (check_cpp_feature)


-- Configuring incomplete, errors occurred!

This check is as generic as possible so we can easily add/remove features from this list.