[dmd-cxx] Add root/system.h header for wrapping system includes

[ibuclaw]

Portability of the front-end is not well covered for older targets.

For instance, while Solaris 11 can build just fine, Solaris 10 fails with:

/vol/gcc/src/hg/trunk/local/gcc/d/dmd/arrayop.c: In member function 'virtual void buildArrayIdent(Expression*, OutBuffer*, Expressions*)::BuildArrayIdentVisitor::visit(BinAssignExp*)':
/vol/gcc/src/hg/trunk/local/gcc/d/dmd/arrayop.c:336:29: error: 's' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  336 |             buf->writestring(s);
      |             ~~~~~~~~~~~~~~~~^~~

/vol/gcc/src/hg/trunk/local/gcc/d/dmd/ctfeexpr.c: In function 'int comparePointers(TOK, Expression*, dinteger_t, Expression*, dinteger_t)':
/vol/gcc/src/hg/trunk/local/gcc/d/dmd/ctfeexpr.c:925:13: error: 'n' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  925 |         int n;
      |     

This is because assert is not marked as noreturn in <assert.h>.

GCC itself provides a couple of headers that wrap around vagaries of the host system to provide a consistent interface for maximum portability.

This patch then moves most front-end system includes into a central place, allowing gdc to override and provide its own copy of this header that includes the gcc headers, or provide gcc-specific portability fixes to the C++ sources.

I need to first check this against gdc to make sure it actually fits well.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "dmd-cxx + dmd#8910"

[ibuclaw]

All malloc and friends are all marked as #pragma GCC poison, so have added ifdefs to forward to the correct xmalloc and friends if compiling gdc.

[ibuclaw]

Rebased.

[ibuclaw]

@jacob-carlborg - I'll just split the latter two, they don't touch any part of the shareable front-end anyway.

Everything else ok?

[jacob-carlborg]

Everything else ok?

Looks good, nothing else that I can see.