meson/setup-hook.sh: make build directory configurable

doc/hooks/meson.section.md

···
 
 Controls the flags passed to `meson setup` during configure phase.
 
+#### `mesonBuildDir` {#meson-build-dir}
+
+Directory where Meson will put intermediate files.
+
+Setting this can be useful for debugging multiple Meson builds while in the same source directory, for example, when building for different platforms.
+Different values for each build will prevent build artefacts from interefering with each other.
+This setting has no tangible effect when running the build in a sandboxed derivation.
+
+The default value is `build`.
+
 #### `mesonWrapMode` {#meson-wrap-mode}
 
 Which value is passed as

pkgs/by-name/me/meson/setup-hook.sh

···
 mesonConfigurePhase() {
     runHook preConfigure
 
+    : ${mesonBuildDir:=build}
+
     local flagsArray=()
 
     if [ -z "${dontAddPrefix-}" ]; then
···
 
     echoCmd 'mesonConfigurePhase flags' "${flagsArray[@]}"
 
-    meson setup build "${flagsArray[@]}"
-    cd build || { echoCmd 'mesonConfigurePhase' "could not cd to build"; exit 1; }
+    meson setup "$mesonBuildDir" "${flagsArray[@]}"
+    cd "$mesonBuildDir" || { echoCmd 'mesonConfigurePhase' "could not cd to $mesonBuildDir"; exit 1; }
 
     if ! [[ -v enableParallelBuilding ]]; then
         enableParallelBuilding=1