Merge pull request #9465 from obsidiansystems/build-dir

Use `buildprefix` in a few more places

(cherry picked from commit b6a3fde6b7a416929553e6be36fc991680ddf9ef)
Change-Id: I2790663fa9f8242ac2db6582b7e421d2fdf42942

1 files changed, 25 insertions, 0 deletions

diff --git a/doc/manual/src/contributing/hacking.md b/doc/manual/src/contributing/hacking.md
index dbe0c82cf..6bea6f727 100644
--- a/doc/manual/src/contributing/hacking.md
+++ b/doc/manual/src/contributing/hacking.md
@@ -146,6 +146,31 @@ $ nix build .#packages.aarch64-linux.default
 Cross-compiled builds are available for ARMv6 (`armv6l-linux`) and ARMv7 (`armv7l-linux`).
 Add more [system types](#system-type) to `crossSystems` in `flake.nix` to bootstrap Nix on unsupported platforms.
 
+### Building for multiple platforms at once
+
+It is useful to perform multiple cross and native builds on the same source tree,
+for example to ensure that better support for one platform doesn't break the build for another.
+In order to facilitate this, Nix has some support for being built out of tree – that is, placing build artefacts in a different directory than the source code:
+
+1. Create a directory for the build, e.g.
+
+   ```bash
+   mkdir build
+   ```
+
+2. Run the configure script from that directory, e.g.
+
+   ```bash
+   cd build
+   ../configure <configure flags>
+   ```
+
+3. Run make from the source directory, but with the build directory specified, e.g.
+
+   ```bash
+   make builddir=build <make flags>
+   ```
+
 ## System type
 
 Nix uses a string with he following format to identify the *system type* or *platform* it runs on: