$ git clone --branch master --depth 1 https://github.com/mruby/mruby.git /tmp/mrbgems/mruby
$ cd /tmp/mrbgems/mruby
$ rake
...snip...
CPP   mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.pi
/tmp/mrbgems/mruby/mrbgems/mruby-time/src/time.c:11:10: fatal error: mruby/time.h: No such file or directory
   11 | #include <mruby/time.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
rake aborted!
Command failed with status (1): [gcc -E -P -std=gnu99 -g -O3 -Wall -Wundef ...]
/tmp/mrbgems/mruby/lib/mruby/build/command.rb:33:in `_run'
/tmp/mrbgems/mruby/lib/mruby/build/command.rb:95:in `run'
/tmp/mrbgems/mruby/lib/mruby/build/command.rb:116:in `block (2 levels) in define_rules'
Tasks: TOP => default => all => gensym => /tmp/mrbgems/mruby/build/host/presym => /tmp/mrbgems/mruby/build/host/mrbgems/mruby-time/src/time.pi
(See full trace by running task with --trace)