bpf: check llvm version only when used

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>
(cherry picked from commit c58177b5dcb3461efef0adefe570dd8a8d966ec4)