[PATCH] perf build: Fix build error when NO_LIBBPF=1

[Mike Leach]

Recent updates to perf build result in the following output when cross
compiling to aarch64, with libelf unavailable, and therefore
NO_LIBBPF=1 set.

```
  $make -C tools/perf

  <cut>

  Makefile.config:428: No libelf found. Disables 'probe' tool, jvmti
  and BPF support in 'perf record'. Please install libelf-dev,
  libelf-devel or elfutils-libelf-devel

  <cut>

  libbpf.c:46:10: fatal error: libelf.h: No such file or directory
      46 | #include <libelf.h>
         |          ^~~~~~~~~~
  compilation terminated.

  ./tools/build/Makefile.build:96: recipe for target
  '.tools/perf/libbpf/staticobjs/libbpf.o' failed

```

plus one other include error for <gelf.h>

The issue is that the commit noted below adds libbpf to the prepare:
target but no longer accounts for the NO_LIBBPF define. Additionally
changing the include directories means that even if the libbpf target
build is prevented, bpf headers are missing in other parts of the build.

This patch ensures that in the case of NO_LIBBPF=1, the build target is
changed to a header only target, and the headers are installed, without
attempting to build the libbpf.a target.

Applies to perf/core

Fixes: 746bd29e348f ("perf build: Use tools/lib headers from install path")
Signed-off-by: Mike Leach <mike.leach@xxxxxxxxxx>
---
 tools/perf/Makefile.perf | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 13e7d26e77f0..ee08ecf469f6 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -305,7 +305,11 @@ else
 endif
 LIBBPF_DESTDIR = $(LIBBPF_OUTPUT)
 LIBBPF_INCLUDE = $(LIBBPF_DESTDIR)/include
+ifndef NO_LIBBPF
 LIBBPF = $(LIBBPF_OUTPUT)/libbpf.a
+else
+LIBBPF = $(LIBBPF_INCLUDE)/bpf/bpf.h
+endif
 CFLAGS += -I$(LIBBPF_OUTPUT)/include
 
 ifneq ($(OUTPUT),)
@@ -826,10 +830,16 @@ $(LIBAPI)-clean:
 	$(call QUIET_CLEAN, libapi)
 	$(Q)$(RM) -r -- $(LIBAPI_OUTPUT)
 
+ifndef NO_LIBBPF
 $(LIBBPF): FORCE | $(LIBBPF_OUTPUT)
 	$(Q)$(MAKE) -C $(LIBBPF_DIR) FEATURES_DUMP=$(FEATURE_DUMP_EXPORT) \
 		O= OUTPUT=$(LIBBPF_OUTPUT)/ DESTDIR=$(LIBBPF_DESTDIR) prefix= \
 		$@ install_headers
+else
+$(LIBBPF): FORCE | $(LIBBPF_OUTPUT)
+	$(Q)$(MAKE) -C $(LIBBPF_DIR) OUTPUT=$(LIBBPF_OUTPUT)/ \
+		DESTDIR=$(LIBBPF_DESTDIR) prefix= install_headers
+endif
 
 $(LIBBPF)-clean:
 	$(call QUIET_CLEAN, libbpf)
-- 
2.17.1