remove various preprocessor conditionals

- HAVE_IEEEFP_H is nowhere defined or needed at all
- FreeBSD < 4 is unsupported since years
- MSVC workaround (around what exactly?)
use the same error msg as mrb_str_to_cstr/CRuby

It's better to have one message for the same error
than two different (although similar worded) ones.
Fix extended arena check in gc_protect
Merge pull request #1631 from cremno/fix-ruby-warnings

fix some warnings emitted by 'ruby -w'
Merge pull request #1633 from cremno/nan-boxing-uintptr_t

NaN boxing: use uintptr_t for bit manipulations
Merge pull request #1635 from cremno/cpp-conditionals

remove various preprocessor conditionals
Merge pull request #1636 from cremno/mrb_get_args-z-errmsg

use the same error msg as mrb_str_to_cstr/CRuby
Merge pull request #1637 from h2so5/extended-arena-size

Fix extended arena check in gc_protect
Merge pull request #1632 from cremno/superfluous-return

remove superfluous return in codedump_all
Merge pull request #1634 from cremno/use-mrb_fixnum_p

tiny word boxing optimization
use macros for assignments to ci->err
mrb_ary_splat() to call #to_a
Merge pull request #1639 from h2so5/macro-ci-err

use macros for assignments to ci->err
Merge branch 'master' of github.com:mruby/mruby
quicker release of arena after ensure calls
always clear ci->err
pop ensure stack before execution; close #1638
should splat and iterate elements when "*ary" speficied in the case-when clause; close #1627
object_id may not return Fixnum (ISO says Integer), since intptr_t may be bigger than Fixnum, so type check for Numeric; ref #1630
Test case statement with splat