Use CMake to generate --help strings from markdown docs.

cmake/FileToString.cmake

@@ -0,0 +1,19 @@
+# file_to_string(file_in file_out string_name)
+#
+# Create a C++ file `file_out` that defines a string named `string_name` in
+# `namespace`, which contains the contents of `file_in`.
+
+function(file_to_string file_in file_out namespace string_name)
+  file(READ "${file_in}" file_in_contents)
+
+  set(file_out_contents)
+  string(APPEND file_out_contents
+    "#include <string>\n"
+    "namespace ${namespace} {\n"
+    "const std::string ${string_name} =\n"
+    "R\"expected(${file_in_contents})expected\";\n"
+    "}\n"
+  )
+
+  file(WRITE "${file_out}" "${file_out_contents}")
+endfunction()