libbpf: Build out-of-tree

debian/changelog

@@ -4,6 +4,7 @@ linux (4.19.37-4) UNRELEASED; urgency=medium
     - Drop unnecessary changes from "libbpf: add SONAME to shared object"
     - libbpf: Use only 2 components in soversion, matching package name
       (Closes: #929187)
+    - libbpf: Build out-of-tree
 
  -- Ben Hutchings <ben@decadent.org.uk>  Sun, 19 May 2019 00:04:16 +0100
 

debian/rules.d/tools/lib/bpf/Makefile

@@ -5,7 +5,7 @@ DEB_HOST_MULTIARCH := $(shell dpkg-architecture -qDEB_HOST_MULTIARCH)
 # Catch use of missing kernel APIs early
 CFLAGS += -Werror=implicit-function-declaration
 
-MAKE_BPF := +$(MAKE) -C $(top_srcdir)/$(OUTDIR)
+MAKE_BPF := +$(MAKE) -C $(top_srcdir)/$(OUTDIR) O=$(CURDIR)
 MAKE_BPF += prefix=/usr
 MAKE_BPF += libdir_relative=lib/$(DEB_HOST_MULTIARCH)
 MAKE_BPF += V=$(KBUILD_VERBOSE)