Use buildprefix in a few more places

Makefile

@@ -1,5 +1,7 @@
--include Makefile.config
-clean-files += Makefile.config
+include mk/build-dir.mk
+
+-include $(buildprefix)Makefile.config
+clean-files += $(buildprefix)Makefile.config
 
 ifeq ($(ENABLE_BUILD), yes)
 makefiles = \

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:

mk/build-dir.mk

@@ -0,0 +1,10 @@
+# Initialise support for build directories.
+builddir ?=
+
+ifdef builddir
+  buildprefix = $(builddir)/
+  buildprefixrel = $(builddir)
+else
+  buildprefix =
+  buildprefixrel = .
+endif

mk/install-dirs.mk

@@ -0,0 +1,11 @@
+# Default installation paths.
+prefix ?= /usr/local
+libdir ?= $(prefix)/lib
+bindir ?= $(prefix)/bin
+libexecdir ?= $(prefix)/libexec
+datadir ?= $(prefix)/share
+localstatedir ?= $(prefix)/var
+sysconfdir ?= $(prefix)/etc
+mandir ?= $(prefix)/share/man
+
+DESTDIR ?=

mk/lib.mk

@@ -43,27 +43,6 @@ define newline
 endef
 
 
-# Default installation paths.
-prefix ?= /usr/local
-libdir ?= $(prefix)/lib
-bindir ?= $(prefix)/bin
-libexecdir ?= $(prefix)/libexec
-datadir ?= $(prefix)/share
-localstatedir ?= $(prefix)/var
-sysconfdir ?= $(prefix)/etc
-mandir ?= $(prefix)/share/man
-
-
-# Initialise support for build directories.
-builddir ?=
-
-ifdef builddir
-  buildprefix = $(builddir)/
-else
-  buildprefix =
-endif
-
-
 # Pass -fPIC if we're building dynamic libraries.
 BUILD_SHARED_LIBS ?= 1
 
@@ -94,6 +73,8 @@ ifeq ($(BUILD_DEBUG), 1)
 endif
 
 
+include mk/build-dir.mk
+include mk/install-dirs.mk
 include mk/functions.mk
 include mk/tracing.mk
 include mk/clean.mk
@@ -112,7 +93,7 @@ define include-sub-makefile
   include $(1)
 endef
 
-$(foreach mf, $(makefiles), $(eval $(call include-sub-makefile, $(mf))))
+$(foreach mf, $(makefiles), $(eval $(call include-sub-makefile,$(mf))))
 
 
 # Instantiate stuff.

mk/templates.mk

@@ -10,10 +10,10 @@ endef
 
 ifneq ($(MAKECMDGOALS), clean)
 
-%.h: %.h.in
-	$(trace-gen) rm -f $@ && ./config.status --quiet --header=$@
+$(buildprefix)%.h: %.h.in
+	$(trace-gen) rm -f $@ && cd $(buildprefixrel) && ./config.status --quiet --header=$(@:$(buildprefix)%=%)
 
-%: %.in
-	$(trace-gen) rm -f $@ && ./config.status --quiet --file=$@
+$(buildprefix)%: %.in
+	$(trace-gen) rm -f $@ && cd $(buildprefixrel) && ./config.status --quiet --file=$(@:$(buildprefix)%=%)
 
 endif

src/libcmd/local.mk

@@ -12,4 +12,4 @@ libcmd_LDFLAGS = $(EDITLINE_LIBS) $(LOWDOWN_LIBS) -pthread
 
 libcmd_LIBS = libstore libutil libexpr libmain libfetchers
 
-$(eval $(call install-file-in, $(d)/nix-cmd.pc, $(libdir)/pkgconfig, 0644))
+$(eval $(call install-file-in, $(buildprefix)$(d)/nix-cmd.pc, $(libdir)/pkgconfig, 0644))

src/libexpr/local.mk

@@ -36,7 +36,7 @@ $(d)/lexer-tab.cc $(d)/lexer-tab.hh: $(d)/lexer.l
 
 clean-files += $(d)/parser-tab.cc $(d)/parser-tab.hh $(d)/lexer-tab.cc $(d)/lexer-tab.hh
 
-$(eval $(call install-file-in, $(d)/nix-expr.pc, $(libdir)/pkgconfig, 0644))
+$(eval $(call install-file-in, $(buildprefix)$(d)/nix-expr.pc, $(libdir)/pkgconfig, 0644))
 
 $(foreach i, $(wildcard src/libexpr/value/*.hh), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix/value, 0644)))
@@ -47,4 +47,4 @@ $(d)/primops.cc: $(d)/imported-drv-to-derivation.nix.gen.hh
 
 $(d)/eval.cc: $(d)/primops/derivation.nix.gen.hh $(d)/fetchurl.nix.gen.hh $(d)/flake/call-flake.nix.gen.hh
 
-src/libexpr/primops/fromTOML.o:	ERROR_SWITCH_ENUM =
+$(buildprefix)src/libexpr/primops/fromTOML.o:	ERROR_SWITCH_ENUM =

src/libmain/local.mk

@@ -14,4 +14,4 @@ libmain_LIBS = libstore libutil
 
 libmain_ALLOW_UNDEFINED = 1
 
-$(eval $(call install-file-in, $(d)/nix-main.pc, $(libdir)/pkgconfig, 0644))
+$(eval $(call install-file-in, $(buildprefix)$(d)/nix-main.pc, $(libdir)/pkgconfig, 0644))

src/libstore/local.mk

@@ -59,7 +59,7 @@ $(d)/build.cc:
 
 clean-files += $(d)/schema.sql.gen.hh $(d)/ca-specific-schema.sql.gen.hh
 
-$(eval $(call install-file-in, $(d)/nix-store.pc, $(libdir)/pkgconfig, 0644))
+$(eval $(call install-file-in, $(buildprefix)$(d)/nix-store.pc, $(libdir)/pkgconfig, 0644))
 
 $(foreach i, $(wildcard src/libstore/builtins/*.hh), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix/builtins, 0644)))

tests/functional/local.mk

@@ -140,9 +140,9 @@ ifeq ($(ENABLE_BUILD), yes)
 endif
 
 $(d)/test-libstoreconsumer.sh.test $(d)/test-libstoreconsumer.sh.test-debug: \
-  $(d)/test-libstoreconsumer/test-libstoreconsumer
+  $(buildprefix)$(d)/test-libstoreconsumer/test-libstoreconsumer
 $(d)/plugins.sh.test $(d)/plugins.sh.test-debug: \
-  $(d)/plugins/libplugintest.$(SO_EXT)
+  $(buildprefix)$(d)/plugins/libplugintest.$(SO_EXT)
 
 install-tests += $(foreach x, $(nix_tests), $(d)/$(x))