bpf: set gcc std and compile flags

This should make gcc bpf compilation more reliable.

src/core/bpf/meson.build

@@ -5,6 +5,7 @@ if conf.get('BPF_FRAMEWORK') != 1
 endif
 
 bpf_clang_flags = [
+        '-std=gnu11',
         '-Wno-compare-distinct-pointer-types',
         '-O2',
         '-target',
@@ -14,11 +15,13 @@ bpf_clang_flags = [
 ]
 
 bpf_gcc_flags = [
+        '-std=gnu11',
         '-O2',
         '-mkernel=5.2',
         '-mcpu=v3',
         '-mco-re',
         '-gbtf',
+        '-c',
 ]
 
 # Generate defines that are appropriate to tell the compiler what architecture