bpf: check llvm version only when used

unetd always includes $(INCLUDE_DIR)/bpf.mk. This file always checks if
the LLVM version is supported in CLANG_VER_VALID. unetd only needs bpf
when UNETD_VXLAN_SUPPORT is set. It fails when UNETD_VXLAN_SUPPORT is
not set and llvm is not installed.

Fix it by only checking the LLVM version when a LLVM toolchain is
available.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

include/bpf.mk

@@ -63,6 +63,7 @@ BPF_CFLAGS := \
 	-Wno-unused-label \
 	-O2 -emit-llvm -Xclang -disable-llvm-passes
 
+ifneq ($(CONFIG_HAS_BPF_TOOLCHAIN),)
 ifeq ($(DUMP),)
   CLANG_VER:=$(shell $(CLANG) -dM -E - < /dev/null | grep __clang_major__ | cut -d' ' -f3)
   CLANG_VER_VALID:=$(shell [ "$(CLANG_VER)" -ge "$(CLANG_MIN_VER)" ] && echo 1 )
@@ -70,6 +71,7 @@ ifeq ($(DUMP),)
     $(error ERROR: LLVM/clang version too old. Minimum required: $(CLANG_MIN_VER), found: $(CLANG_VER))
   endif
 endif
+endif
 
 define CompileBPF
 	$(CLANG) -g -target $(BPF_ARCH)-linux-gnu $(BPF_CFLAGS) $(2) \