Add libbpf as a submodule

This is to potentinally reduce issues with folks using
different versions of libbpf at runtime.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "libbpf"]
+	path = libbpf
+	url = https://github.com/libbpf/libbpf

README.md

@@ -198,7 +198,7 @@ repo](https://mesonbuild.com/Quick-guide.html#installation-from-source) and call
   standard utilities including `awk`.
 - `clang`: >=16 required, >=17 recommended
 - `libbpf`: >=1.2.2 required, >=1.3 recommended (`RESIZE_ARRAY` support is
-  new in 1.3)
+  new in 1.3). It's preferred to link statically against the source from the libbpf git submodule.
 - Rust toolchain: >=1.72
 - `libelf`, `libz`, `libzstd` if linking against staic `libbpf.a`
 - `bpftool` (usually available in `linux-tools-common`)
@@ -210,9 +210,21 @@ repo](https://mesonbuild.com/Quick-guide.html#installation-from-source) and call
 commands in the root of the tree builds and installs all schedulers under
 `~/bin`.
 
+#### Static linking against libbpf submodule (preferred)
+
+ ```
+$ cd $SCX
+$ git submodule init
+$ make -C libbpf/src
+$ meson setup build --prefix ~
+$ meson compile -C build
+$ meson install -C build
+```
+
+#### Dynamic linking against libbpf
 ```
 $ cd $SCX
-$ meson setup build --prefix ~
+$ meson setup build --prefix ~ -D libbpf_a=disabled
 $ meson compile -C build
 $ meson install -C build
 ```

libbpf

@@ -0,0 +1 @@
+Subproject commit 20c0a9e3d7e7d4aeb283eae982543c9cacc29477

meson.build

@@ -44,9 +44,21 @@ elif bpf_clang_maj < 17
           .format(bpf_clang.full_path(), bpf_clang_ver))
 endif
 
-if get_option('libbpf_a') != ''
+libbpf_a = '@0@/libbpf/src/libbpf.a'.format(meson.current_source_dir())
+
+if get_option('libbpf_a') == 'disabled'
+  libbpf_a = ''
+elif get_option('libbpf_a') != ''
+  libbpf_a = get_option('libbpf_a')
+endif
+
+if libbpf_a != ''
+  if not fs.exists(libbpf_a)
+    error('@0@ does not exist. You need to build/make libbpf.'.format(libbpf_a))
+  endif
+
   libbpf_dep = [declare_dependency(
-    link_args: get_option('libbpf_a'),
+    link_args: libbpf_a,
     include_directories: get_option('libbpf_h')),
     cc.find_library('elf'),
     cc.find_library('z'),
@@ -98,7 +110,7 @@ message('cpu=@0@ bpf_base_cflags=@1@'.format(cpu, bpf_base_cflags))
 
 libbpf_c_headers = []
 
-if get_option('libbpf_a') != ''
+if libbpf_a != ''
   foreach header: get_option('libbpf_h')
     libbpf_c_headers += ['-I', header]
   endforeach
@@ -134,14 +146,14 @@ foreach flag: bpf_base_cflags
   cargo_env.append('BPF_BASE_CFLAGS', flag, separator: ' ')
 endforeach
 
-if get_option('libbpf_a') != ''
+if libbpf_a != ''
   foreach header: get_option('libbpf_h')
     cargo_env.append('BPF_EXTRA_CFLAGS_PRE_INCL', '-I' + header, separator: ' ')
   endforeach
 
   cargo_env.append('RUSTFLAGS',
                    '-C link-args=-lelf -C link-args=-lz -C link-args=-lzstd -L '
-                   + fs.parent(get_option('libbpf_a')))
+                   + fs.parent(libbpf_a))
 
   #
   # XXX - scx_rusty's original Cargo.toml contained a dependency matching

meson.options

@@ -3,7 +3,7 @@ option('bpf_clang', type: 'string', value: 'clang',
 option('bpftool', type: 'string', value: 'bpftool',
        description: 'bpftool to use when generating .bpf.skel.h')
 option('libbpf_a', type: 'string',
-       description: 'Static libbpf.a to use')
+       description: 'Static libbpf.a to use. Default is to use the one inside the libbpf submodule. Set this option to "disabled" to link libbpf dynamically.')
 option('libbpf_h', type: 'array',
        description: 'libbpf header directories, only meaningful with libbpf_a option')
 option('cargo', type: 'string', value: 'cargo',