Initialize CFLAGS to the default value before including
tools/scripts/Makefile.include.

Defer appending EXTRA_CFLAGS to CFLAGS until after including
Makefile.include, as it may extend EXTRA_CFLAGS in the future.

Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 tools/lib/bpf/Makefile | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index 168140f8e6461bd06db40e23d21a3fb8847ccbf4..76cc802375f67b9d8f589904bd9764040947e5dd 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -49,6 +49,14 @@ man_dir_SQ = '$(subst ','\'',$(man_dir))'
 export man_dir man_dir_SQ INSTALL
 export DESTDIR DESTDIR_SQ
 
+# Defer assigning EXTRA_CFLAGS to CFLAGS until after including
+# tools/scripts/Makefile.include, as it may add flags to EXTRA_CFLAGS.
+ifdef EXTRA_CFLAGS
+  CFLAGS :=
+else
+  CFLAGS := -g -O2
+endif
+
 include $(srctree)/tools/scripts/Makefile.include
 
 # copy a bit from Linux kbuild
@@ -70,14 +78,8 @@ LIB_TARGET	= libbpf.a libbpf.so.$(LIBBPF_VERSION)
 LIB_FILE	= libbpf.a libbpf.so*
 PC_FILE		= libbpf.pc
 
-# Set compile option CFLAGS
-ifdef EXTRA_CFLAGS
-  CFLAGS := $(EXTRA_CFLAGS)
-else
-  CFLAGS := -g -O2
-endif
-
 # Append required CFLAGS
+override CFLAGS += $(EXTRA_CFLAGS)
 override CFLAGS += -std=gnu89
 override CFLAGS += $(EXTRA_WARNINGS) -Wno-switch-enum
 override CFLAGS += -Werror -Wall

-- 
2.34.1