[PATCH bpf-next] selftests/bpf: Fix cross compilation with CLANG_CROSS_FLAGS

[Florent Revest]

I cross-compile my BPF selftests with the following command:

CLANG_CROSS_FLAGS="--target=aarch64-linux-gnu --sysroot=/sysroot/" \
  make LLVM=1 CC=clang CROSS_COMPILE=aarch64-linux-gnu- SRCARCH=arm64

(Note the use of CLANG_CROSS_FLAGS to specify a custom sysroot instead
of letting clang use gcc's default sysroot)

However, CLANG_CROSS_FLAGS gets propagated to host tools builds (libbpf
and bpftool) and because they reference it directly in their Makefiles,
they end up cross-compiling host objects which results in linking
errors.

This patch ensures that CLANG_CROSS_FLAGS is reset if CROSS_COMPILE
isn't set (for example when reaching a BPF host tool build).

Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
---
 tools/scripts/Makefile.include | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 0efb8f2b33ce..ff527ac065cf 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -108,6 +108,8 @@ endif # GCC_TOOLCHAIN_DIR
 endif # CLANG_CROSS_FLAGS
 CFLAGS += $(CLANG_CROSS_FLAGS)
 AFLAGS += $(CLANG_CROSS_FLAGS)
+else
+CLANG_CROSS_FLAGS :=
 endif # CROSS_COMPILE
 
 # Hack to avoid type-punned warnings on old systems such as RHEL5:
-- 
2.39.2.637.g21b0678d19-goog