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>