meson: Define our own clang-tidy target

meson's target has a few issues:
- Runs on all source files regardless if they're included in the
  build or not
- Doesn't have any dependencies on generated sources which means we
  have to do a full build first before we can run clang-tidy
- Doesn't allow us to pass any extra arguments

To work around these, let's define our own clang-tidy target instead
using llvm's run-clang-tidy script. Alongside the clang-tidy target,
let's start keeping track of all generated sources which we make the
clang-tidy target depend on. We also add a new target which will only
generate source files which is useful for setting up the source tree
for running code analysis against it.

src/nspawn/meson.build

@@ -20,7 +20,8 @@ nspawn_gperf_c = custom_target(
         output : 'nspawn-gperf.c',
         command : [gperf, '@INPUT@', '--output-file', '@OUTPUT@'])
 
-libnspawn_core_sources += [nspawn_gperf_c]
+generated_sources += nspawn_gperf_c
+libnspawn_core_sources += nspawn_gperf_c
 
 libnspawn_core = static_library(
         'nspawn-core',