more mrb_flo_to_str fixes

- remove float check (take mrb_float instead of mrb_value)
- support -0.0
- fix range error if log10 is called when n == 0.0
- MSVC older than 2013: add signbit macro (for double)
mrb_flo_to_str: internal linkage
use mrb_intern_lit in mruby-random
add 'd' format specifier to get data pointer directly from mrb_get_args
use 'd' format spec to get Random object
fix possible arena overflow
add tests for String#capitalize! and String#downcase!
add test(requires MRB_GC_FIXED_ARENA enabled)
fix the position of capitalize! and downcast!, and add test for upcase!
Merge pull request #1723 from akuroda/string_test_cap_down

add tests for String#capitalize!, String#downcase!, and String#upcase!
Merge pull request #1722 from take-cheeze/hash_ext_arena_fix

Fix possible arena overflow in mruby-hast-ext.
powered num may be infinite in float-string conversion
preserve significands in float-string conversion
Merge pull request #1724 from h2so5/f2s-significand

preserve significands in float-string conversion
add src/error.h for compatibility reason
Add warning for C89-style variable declarations
make mruby build directory customizable
move declaration to the top of the block; ref #1727
Fix unused function warning when readline enabled (mirb)
support custom build_dir in CrossBuild