Squashed 'external/mio/' content from commit 8b6b7d8

git-subtree-dir: external/mio
git-subtree-split: 8b6b7d878c89e81614d05edca7936de41ccdd2da

include/mio/CMakeLists.txt

@@ -0,0 +1,11 @@
+#
+# While not strictly necessary to specify header files as target sources,
+# doing so populates these files in the source listing when CMake is used
+# to generate XCode and Visual Studios projects
+#
+target_sources(mio-headers INTERFACE
+  "${prefix}/mio/mmap.hpp"
+  "${prefix}/mio/page.hpp"
+  "${prefix}/mio/shared_mmap.hpp")
+
+add_subdirectory(detail)