Pacify MSVC warnings for numeric.c, proc.c, and symbol.c
Change mrblib's source path to absolute path
Merge pull request #2600 from mimaki/fix_msvc_warnings

Pacify MSVC warnings for numeric.c, proc.c, and symbol.c
cast MRB_ENV_STACK_LEN to (mrb_int); ref #2600
Merge pull request #2601 from mimaki/mrblib_absolute_path

Change mrblib's source path to absolute path