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
clarify fallthrough in the switch statement
specialize C++ compiler in clang toolchain
fix missing spaces.
resolve conflict
Merge branch 'master' of github.com:mruby/mruby
Update README.md; The URL of the mruby home-page is accessible.
Merge pull request #1740 from tmash06/fix_missing_spaces

fix missing spaces.
Merge pull request #1741 from suzukaze/update-readme

Update README.md; The URL of the mruby home-page is accessible.
Merge pull request #1739 from take-cheeze/clang_cxx

Specialize C++ compiler in clang toolchain.
use mrb_str_new_lit instead of mrb_str_new for C string literals
introduce mrb_str_cat_lit() to create strings from C string litrals
use mrb_str_cat_lit() more widely
use mrb_intern_lit instead of mrb_intern_cstr for C string literals
fix bug for get_valid_iv_sym

When sub-string is used in the following way, it does not work well.

o = Object.new
o.instance_variable_set(:@a, 1)
o.instance_variable_defined?("@abc"[0,2]) #=> false (this should be true)
add test for __LINE__ and __FILE__
Merge pull request #1743 from cubicdaiya/issues/use_mrb_str_new_lit

Use mrb_str_new_lit instead of mrb_str_new for C string literals
add macro ARY_SHARED_P
Merge pull request #1744 from cubicdaiya/feature/introduce_mrb_str_cat_lit

Introduce mrb_str_cat_lit