build: Split up features into separate libraries

This makes it much more obvious when a feature is missing for a user.

Change-Id: Ibb17d7ab1f185a1976a32f48933c01a252450dd1
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include-uring/meson.build b/include-uring/meson.build
new file mode 100644
index 0000000..91b426f
--- /dev/null
+++ b/include-uring/meson.build
@@ -0,0 +1,5 @@
+stdplus_headers += include_directories('.')
+
+install_headers(
+  'stdplus/io_uring.hpp',
+  subdir: 'stdplus')