GCC: add -std=gnu99 to CFLAGS

- this change prevents issues like #1539 in the future
- Clang's default is already gnu99 but GCC's is gnu89
- mruby is C99 (+ GNU exts), not C89 (+ GNU exts)

irep->pool[n].type should be IREP_TT_*, not MRB_TT_*

fix warning: "implicit conversion from enumeration type 'enum mrb_vtype' to different enumeration type 'enum irep_pool_type' [-Wenum-conversion]"

fix length check in read_section_debug()

Changed a variable declaration to top of block

Change supress to suppress

nextc(): always return an '\n' at end of input

fix #1564