Export CC to the test shell environment

[wilzbach]

Useful for e.g. #8342

This performs what d_do_test already does today:

dmd/test/tools/d_do_test.d

Lines 588 to 596 in cce909b

	if (envData.ccompiler.empty)
	{
	switch (envData.os)
	{
	case "win32": envData.ccompiler = "dmc"; break;
	case "win64": envData.ccompiler = `\"Program Files (x86)"\"Microsoft Visual Studio 10.0"\VC\bin\amd64\cl.exe`; break;
	default: envData.ccompiler = "c++"; break;
	}
	}

(though the win64 case isn't really used anymore)

[dlang-bot]

Thanks for your pull request, @wilzbach!

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 "master + dmd#8343"

[wilzbach]

So we managed to reach the state where CC refers to the C++ compiler:

dmd/test/tools/d_do_test.d

Lines 588 to 596 in cce909b

	if (envData.ccompiler.empty)
	{
	switch (envData.os)
	{
	case "win32": envData.ccompiler = "dmc"; break;
	case "win64": envData.ccompiler = `\"Program Files (x86)"\"Microsoft Visual Studio 10.0"\VC\bin\amd64\cl.exe`; break;
	default: envData.ccompiler = "c++"; break;
	}
	}

dmd/test/README.md

Line 188 in cce909b

CC: C++ compiler to use, ex: dmc, g++

But in this case CC is used as people normally use it: the C compiler.

In this case it makes a difference whether a C or C++ compiler is used.
I would suggest we introduce a new environment variable (e.g. CPP) and let CC point to the system's C compiler.
FWIW in the build make scripts the C++ compiler is at least called HOST_CXX.