Use buildprefix in a few more places

`installcheck` doesn't yet work, but the rest of the build can now
happen mostly inside a separate build directory.

Progress on #9342

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

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.