doc: compile tutorial programs

The programs in `doc/examples` are not compiled currently, so let's
compile them if the `docs` and `examples` options don't disallow it.

`tutorial4.c` needs a small modification to avoid `-Wfloat-conversion`.

Additionally, install them if `installed_tests` is not disabled.

doc/examples/meson.build

@@ -0,0 +1,23 @@
+tutorials = [
+  'tutorial1',
+  'tutorial2',
+  'tutorial3',
+  'tutorial4',
+  'tutorial5',
+  'tutorial6',
+  'tutorial7',
+]
+
+if not get_option('examples').allowed()
+  subdir_done()
+endif
+
+foreach c : tutorials
+  executable(
+    c,
+    sources: c + '.c',
+    dependencies: [ pipewire_dep, mathlib ],
+    install: installed_tests_enabled,
+    install_dir: installed_tests_execdir / 'examples',
+  )
+endforeach

doc/examples/tutorial4.c

@@ -56,7 +56,7 @@ static void on_process(void *userdata)
 		 * Another common method to convert a double to
 		 * 16 bits is to multiple by 32768.0 and then clamp to
 		 * [-32768 32767] to get the full 16 bits range. */
-		val = sin(data->accumulator) * DEFAULT_VOLUME * 32767.0;
+		val = (int16_t) (sin(data->accumulator) * DEFAULT_VOLUME * 32767.0);
 		for (c = 0; c < DEFAULT_CHANNELS; c++)
 			*dst++ = val;
 	}

doc/meson.build

@@ -169,18 +169,14 @@ cssfiles = [
 ]
 
 # Example files (in order from simple to esoteric)
-example_files = [
-  'tutorial1.c',
-  'tutorial2.c',
-  'tutorial3.c',
-  'tutorial4.c',
-  'tutorial5.c',
-  'tutorial6.c',
-  'tutorial7.c',
-]
+example_files = []
 example_dep_files = []
-foreach h : example_files
-  example_dep_files += ['examples/' + h]
+
+subdir('examples')
+
+foreach h : tutorials
+  example_files += [h + '.c']
+  example_dep_files += ['examples/' + h + '.c']
 endforeach
 foreach h : examples
   example_files += [h + '.c']