implicit declaration of function ‘mrb_load_fail’

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using standard build_config.rb of master head (commit c3188cac431225fda48718f309ad3d9318a6e44f) I add conf.gem :github => 'mattn/mruby-require' and get the following error:

make
```
...
CC build/host/mrbgems/gem_init.c -> build/host/mrbgems/gem_init.o
CC build/mrbgems/mruby-require/src/mrb_require.c -> build/host/mrbgems/mruby-require/src/mrb_require.o
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c: In function ‘find_file’:
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c:234:3: error: implicit declaration of function ‘mrb_load_fail’; did you mean ‘mrb_load_file’? [-Werror=implicit-function-declaration]
mrb_load_fail(mrb, filename, "cannot load such file");
^~~~~~~~~~~~~
mrb_load_file
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c: In function ‘mrb_load_irep_data’:
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c:318:5: error: implicit declaration of function ‘replace_stop_with_return’ [-Werror=implicit-function-declaration]
replace_stop_with_return(mrb, irep);
^~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c: In function ‘find_file’:
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c:234:3: error: implicit declaration of function ‘mrb_load_fail’; did you mean ‘mrb_load_file’? [-Werror=implicit-function-declaration]
mrb_load_fail(mrb, filename, "cannot load such file");
^~~~~~~~~~~~~
mrb_load_file
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c: In function ‘mrb_load_irep_data’:
/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c:318:5: error: implicit declaration of function ‘replace_stop_with_return’ [-Werror=implicit-function-declaration]
replace_stop_with_return(mrb, irep);
^~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
rake aborted!
Command Failed: [gcc -g -std=gnu99 -O3 -Wall -Werror-implicit-function-declaration -Wdeclaration-after-statement -Wwrite-strings -DMRBGEMS_ROOT=\"/opt/mruby/build/test/lib\" -DMRBGEM_MRUBY_REQUIRE_VERSION=0.0.0 -I"/opt/mruby/include" -I"/opt/mruby/src" -MMD -o "/opt/mruby/build/host/mrbgems/mruby-require/src/mrb_require.o" -c "/opt/mruby/build/mrbgems/mruby-require/src/mrb_require.c"]

Makefile:8: recipe for target 'all' failed
make: *** [all] Error 1
```

Start in build/mrbgems/mruby-require/src/mrb_require.c, focusing on find_file and mrb_load_irep_data where mrb_load_fail and replace_stop_with_return are called. Reproduce the reported build with the standard build_config.rb, then inspect the corresponding headers or APIs. Done means the mruby-require gem builds without implicit-declaration errors.

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