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
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
Merge pull request #1728 from kyab/fix_unused_error_mirb

Fix unused function warning when readline enabled (mirb)
Merge pull request #1729 from take-cheeze/customizable_build_dir

Make mruby build directory customizable.
Merge pull request #1727 from bggd/patch-2

Add warning for C89-style variable declarations
limit preserved significand length
remove -Werror=declaration-after-statement
add -Werror=declaration-after-statement
Issue minor corrections and updates to INSTALL file.
Always include build_config.rb to compile dependency.

build_config.rb modifies `cc.defines` (eg. `DISABLE_GEMS`) so files depending on such flags should be rebuilt.

Ref. (Failing scenario): https://gist.github.com/cho45/9181191
Merge pull request #1730 from bggd/patch-3

Add error for C99 style variable declaration on travis-ci
Merge pull request #1731 from jeremyong/master

Issue minor corrections and updates to INSTALL file.