build: Add CMake-based build system

Co-authored-by: Tim Ruffing <crypto@timruffing.de>

cmake/CheckStringOptionValue.cmake

@@ -0,0 +1,12 @@
+function(check_string_option_value option)
+  get_property(expected_values CACHE ${option} PROPERTY STRINGS)
+  if(expected_values)
+    foreach(value IN LISTS expected_values)
+      if(value STREQUAL "${${option}}")
+        return()
+      endif()
+    endforeach()
+    message(FATAL_ERROR "${option} value is \"${${option}}\", but must be one of ${expected_values}.")
+  endif()
+  message(AUTHOR_WARNING "The STRINGS property must be set before invoking `check_string_option_value' function.")
+endfunction()