add derivation parser benchmark

the current identified bottlenecks are parseString in derivations.cc and dirOf (because of std::filessystem creation).

src/libstore-tests/meson.build

@@ -105,3 +105,19 @@ test(
   },
   protocol : 'gtest',
 )
+
+# Build benchmarks if enabled
+if get_option('benchmarks')
+  gbenchmark = dependency('benchmark', required : true)
+
+  benchmark_exe = executable(
+    'nix-store-benchmarks',
+    'derivation-parser-bench.cc',
+    config_priv_h,
+    dependencies : deps_private_subproject + deps_private + deps_other + [gbenchmark],
+    include_directories : include_dirs,
+    link_args: linker_export_flags,
+    install : false,
+    cpp_args : ['-DNIX_UNIT_TEST_DATA="' + meson.current_source_dir() + '/data"'],
+  )
+endif