[#3731] Meson options are now features

Also:
- Add fallback subprojects so that auto_features can take effect.
- Remove -Dall option. auto_features can be used instead.
- Add _dep suffix to dependency names.
- Capitalize dependencies that are used in other meson.build files.

subprojects/postgresql/meson.build

@@ -0,0 +1,18 @@
+project('postgresql')
+postgresql = disabler()
+pg_config = find_program('pg_config', required: false)
+if pg_config.found()
+    cflags = run_command([pg_config, '--cflags'], check: false)
+    libs = run_command([pg_config, '--libs'], check: false)
+    version = run_command([pg_config, '--version'], check: false)
+    if cflags.returncode() == 0 and libs.returncode() == 0 and version.returncode() == 0
+        pgsql_compile_args = cflags.stdout().split()
+        pgsql_link_args = libs.stdout().split()
+        pgsql_version = version.stdout().strip()
+        postgresql = declare_dependency(
+            compile_args: pgsql_compile_args,
+            link_args: pgsql_link_args,
+            version: pgsql_version,
+        )
+    endif
+endif