add Chapter 15 - runtime observer implementation

Chapter15/observer/libs/etl/meson.build

@@ -0,0 +1,27 @@
+project('etl',
+    ['cpp'],
+    default_options : [
+        'warning_level=2',
+        'werror=false',
+        # `build.*` options affect `native: true targets`
+        # plain options affect `native: false` targets.
+        'cpp_std=c++17', 'build.cpp_std=c++17',
+    ],
+    meson_version: '>=0.57.0',
+    version: files('version.txt')
+)
+
+compile_args = []
+if not get_option('use_stl')
+    compile_args += '-DETL_NO_STL'
+endif
+
+etl_dep = declare_dependency(
+    include_directories: include_directories('include'),
+    compile_args: compile_args
+)
+
+# Only register tests with the test runner when built as a primary project
+if meson.is_subproject() == false
+    subdir('test')
+endif