add meson.build for meson build system

Make it possible to include libbpf into projects built by Meson build system as
a subproject. This is the case e.g. for systemd.

meson.build declares dependencies which may be used in parent projects:
`libbpf_static_dep` for static library
`libbpf_shared_dep` for shared library
`bpf_includes` provides the location of bpf.h, btf.h and libbpf.h

Parent projects need to introduce git wrap to build libbpf:
https://github.com/mesonbuild/meson/blob/master/docs/markdown/Wrap-dependency-system-manual.md#wrap-git

An alternative approach is applying a patch with meson.build for libbpf while
building a parent project:
https://github.com/mesonbuild/meson/blob/master/docs/markdown/Wrap-dependency-system-manual.md#wrap-file
Imo it's less transparent and would add more headache.

meson.build is placed in the source root because that's how Meson expects it to
be placed.

The related discussion is in https://github.com/systemd/systemd/pull/12151

README

@@ -43,3 +43,20 @@ headers in a build directory /build/root/:
 
   cd src
   PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make install
+
+To integrate libbpf into a project which uses Meson building system define
+[wrap-git] file in `subprojects` folder.
+To add libbpf dependency to the parent parent project, e.g. for
+libbpf_static_dep:
+
+   libbpf_obj = subproject('libbpf', required : true)
+   libbpf_static_dep = libbpf_proj.get_variable('libbpf_static_dep')
+
+To validate changes to meson.build
+
+   python3 meson.py build
+   ninja -C build/
+
+To install headers, libs and pkgconfig
+   cd build
+   ninja install

meson.build

@@ -0,0 +1,95 @@
+# SPDX-License-Identifier: LGPL-2.1 OR BSD-2-Clause
+project('libbpf', 'c',
+        version : '0.0.3',
+        license : 'LGPL-2.1 OR BSD-2-Clause',
+        default_options : [
+                'prefix=/usr',
+        ],
+        meson_version : '>= 0.46',
+        )
+
+patchlevel = meson.project_version().split('.')[1]
+
+libbpf_source_dir = './'
+
+libbpf_sources = files(run_command('find',
+        [
+        '@0@/src'.format(libbpf_source_dir),
+        '-type',
+        'f',
+        '-name',
+        '*.[h|c]']).stdout().split())
+
+libbpf_headers = files(
+        join_paths(libbpf_source_dir, 'src/bpf.h'),
+        join_paths(libbpf_source_dir, 'src/btf.h'),
+        join_paths(libbpf_source_dir, 'src/libbpf.h'))
+
+feature_rellocarray = run_command(join_paths(libbpf_source_dir, 'scripts/check-reallocarray.sh'))
+
+libbpf_c_args = ['-g',
+                '-O2',
+                '-Werror',
+                '-Wall',
+]
+
+if feature_rellocarray.stdout().strip() != ''
+        libbpf_c_args += '-DCOMPAT_NEED_REALLOCARRAY'
+endif
+
+# bpf_includes are required to include bpf.h, btf.h, libbpf.h
+bpf_includes = include_directories(
+        join_paths(libbpf_source_dir, 'src'))
+
+libbpf_includes = include_directories(
+        join_paths(libbpf_source_dir, 'include'),
+        join_paths(libbpf_source_dir, 'include/uapi'))
+
+libelf = dependency('libelf')
+libelf = dependency('libelf', required: false)
+if not libelf.found()
+        libelf = cc.find_library('elf', required: true)
+endif
+
+deps = [libelf]
+
+libbpf_static = static_library(
+        'bpf',
+        libbpf_sources,
+        c_args : libbpf_c_args,
+        dependencies : deps,
+        include_directories : libbpf_includes,
+        install : true)
+
+libbpf_static_dep = declare_dependency(link_with : libbpf_static)
+
+libbpf_map_source_path = join_paths(libbpf_source_dir, 'src/libbpf.map')
+libbpf_map_abs_path = join_paths(meson.current_source_dir(), libbpf_map_source_path)
+
+libbpf_c_args += ['-fPIC', '-fvisibility=hidden']
+
+libbpf_link_args = ['-Wl,--version-script=@0@'.format(libbpf_map_abs_path)]
+
+libbpf_shared = shared_library(
+        'bpf',
+        libbpf_sources,
+        c_args : libbpf_c_args,
+        dependencies : deps,
+        include_directories : libbpf_includes,
+        install : true,
+        link_args : libbpf_link_args,
+        link_depends : libbpf_map_source_path,
+        soversion : patchlevel,
+        version : meson.project_version())
+
+libbpf_shared_dep = declare_dependency(link_with : libbpf_shared)
+
+install_headers(libbpf_headers, subdir : 'bpf')
+
+pkg = import('pkgconfig')
+pkg.generate(
+        name: meson.project_name(),
+        version: meson.project_version(),
+        libraries: libbpf_shared,
+        requires_private: ['libelf'],
+        description: '''BPF library''')