load.c to use mrb_ro_data_p()
use __init_array_start to determine readonly data section;

b72e94f used _etext and _edata to distinguish C string literals from heap allocated strings,
but using _edata, global char arrays may be considered as string literals.  to avoid the issue,
we have to use __init_array_start, which might be less portable.  you can still use _edata, by
using MRB_NO_INIT_ARRAY_START.
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
fixed. cygwin-gcc(ver4.8.3) warning in conv_digit()
remove accidental check-in of mruby-thread gem
Merge pull request #2602 from Hiroyuki-Matsuzaki/fix_cygwin_gcc_warning

fixed. cygwin-gcc(ver4.8.3) warning in conv_digit()