Document run-make external dependencies

tests/run-make/README.md

@@ -22,5 +22,39 @@ The setup for the `rmake.rs` can be summarized as a 3-stage process:
    and copy non-`rmake.rs` input support files over to `rmake_out/`. The support library is made available as an [*extern prelude*][extern_prelude].
 3. Finally, we run the recipe binary and set `rmake_out/` as the working directory.
 
+## External dependencies
+
+`compiletest` passes tool paths and target-specific flags to `rmake.rs` through
+environment variables. Prefer using the helpers in [`run_make_support`] over
+reading these variables directly. The helpers keep command construction
+consistent across hosts and targets, and avoid relying on whichever tools happen
+to be first in `PATH`.
+
+Commonly used helpers include:
+
+- `rustc()` and `rustdoc()` for the compiler and rustdoc under test, from
+  `RUSTC` and `RUSTDOC`.
+- `cc()` and `cxx()` for the target C and C++ compilers, from `CC`/`CXX` plus
+  `CC_DEFAULT_FLAGS`/`CXX_DEFAULT_FLAGS`.
+- `gcc()` for tests that specifically need `gcc`; unlike `cc()`, this assumes a
+  suitable `gcc` is available in `PATH` and does not add `CC_DEFAULT_FLAGS`.
+- `llvm_ar()`, `llvm_nm()`, `llvm_objdump()`, `llvm_readobj()`, and similar
+  LLVM tools from `LLVM_BIN_DIR`; `llvm_filecheck()` uses `LLVM_FILECHECK`.
+- `python_command()` for the Python interpreter selected by bootstrap, from
+  `PYTHON`.
+- `clang()` for tests that explicitly require clang-based testing, from `CLANG`.
+- `cargo()` for in-tree cargo, from `CARGO`; this is only available in the
+  `run-make-cargo` and `build-std` suites, not in plain `run-make`.
+- `htmldocck()` for the in-tree `src/etc/htmldocck.py` script, invoked through
+  `python_command()`.
+
+Some of these variables are only set when the configured builder can provide the
+corresponding tool. Tests that require optional tools or LLVM target components
+should declare the matching compiletest directive, such as
+`//@ needs-force-clang-based-tests` or `//@ needs-llvm-components: x86`, instead
+of failing later in the recipe. If a test needs a tool that is not represented by
+`run_make_support::external_deps`, add a small helper there rather than open
+coding the lookup in individual tests.
+
 [`run_make_support`]: ../../src/tools/run-make-support
 [extern_prelude]: https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude