Module updates

[justend29]

Problem:

Further use of my previous fixes to the C++ module installation rules revealed a few improvements.

1. Installed Boost::ut_module couldn't be consumed with Clang

   The compilation flags added in WarningsAsErrors.cmake for development leaked into the install
   interface of the Boost::ut and Boost::ut_module targets, inheriting these aggressive flags.
   When using import std, Clang warns about the reserved module name std when compiling std.cc
   from libstdc++ or libc++. With warnings treated as errors, Clang's warningerror about the
   reserved module name prevented Boost::ut_module from being consumed, as it requires compiling
   std.cc. Where Boost::ut was interface-only, these leaky flags wouldn't have an effect, and be easily missed.

   Clang Warning

   [102/199] Building CXX object 'CMakeFiles/@cmake_cxx_std.dir/usr/share/libc++/v1/std.cppm.o'
   /usr/share/libc++/v1/std.cppm:163:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
     163 | export module std;
         |               ^
   1 warning generated.
   [103/199] Building CXX object 'CMakeFiles/@cmake_cxx_std.dir/usr/share/libc++/v1/std.compat.cppm.o'
   /usr/share/libc++/v1/std.compat.cppm:80:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
      80 | export module std.compat;
         |               ^
   1 warning generated.

2. examples/parallel_runner failed to build with Clang

   Tons of linker errors for TBB showed up with clang because parallel_runner was disabled for other compilers. libstdc++
   requires TBB to use <execution>, and the link was missing.

3. Clang warns when angled includes are used in modules.

   The previous PR linked Boost::ut to Boost::ut_module to propagate target properties, including INTERFACE_INCLUDE_DIRECTORIES for header ut.hpp. With this, angled includes were added in ut.cppm, as the public header could be correctly resolved.

   Clang Warning

     [15/35] Building CXX object 'CMakeFiles/Boost__ut_module@synth_0.dir/Debug/440d59585f59.bmi'
     /home/justen/pers-proj/liborbit/build-clang/vcpkg_installed/triplet-clang/include/boost/ut.cppm:12:10: warning: '#include <filename>' attaches the declarations to the named module 'boost.ut', which is not usually intended; consider moving that directive before the module declaration [-Winclude-angled-in-module-purview]
        12 | #include <boost/ut.hpp>
           |          ^
     1 warning generated.

Solution:

1. Wrap the compilation flags for development in the $<BUILD_INTERFACE:> generator expression.
   Another option would be to PRIVATEly add them to each compiled target with
   target_compile_options(<target> PRIVATE <flags>) instead of top-town with
   add_compile_options(<flags>)

2. Link TBB when using libstdc++. TBB has a config-file (TBBConfig.cmake). A simple find_package() does the trick.

3. <boost/ut.hpp> -> "boost/ut.hpp"

   Use quotes to please Clang. Preserve the "boost/" include prefix, as just "ut.hpp" will fail to match system include paths in consuming projects, and emit compiler warnings from the third-party ut.hpp header.

Testing

A full build and test (including from-ut-module) succeeds using GCC + libstdc++, Clang + libstdc++, and Clang + libc++. examples/parallel_runner runs for each of them, too.

Reviewers:
@kris-jusiak