meson: fix missing failure if bpf-framework was enabled

If building with clang and clang does not support bpf, then enabling
-Dbpf-framework=enabled would silently drop the feature (even printing
bpf-framework: enabled in the meson build recap, and no message anywhere
that'd hint at the failure!)

This is unexpected, so add check to fail hard in this case.

All other code paths (gcc, missing bpftool) properly check for the
option, but it is not as easy for a custom command so check explicitly

meson.build

@@ -1102,6 +1102,9 @@ else
                         # Check if 'clang -target bpf' is supported.
                         clang_supports_bpf = run_command(clang, '-target', 'bpf', '--print-supported-cpus', check : false).returncode() == 0
                 endif
+                if bpf_framework.enabled() and not clang_supports_bpf
+                        error('bpf-framework was enabled but clang does not support bpf')
+                endif
         elif bpf_compiler == 'gcc'
                 bpf_gcc = find_program('bpf-gcc',
                                        'bpf-none-gcc',