Define DBL_DIG only if it wasn't previously defined

Fixes #464.

DBL_DIG may already be defined on the target system,
defining it without checking first may overwrite
system values.

Add Test Case for Issue #460

Fix writing errors in Makefiles

compiler may generate broken binary if string literal has NUL characters

Add Tests for current Changes in Code Base

Fixed Windows warning.

Compiling on Windows 7 from a Windows SDK 7 prompt with CMake.
`nmake all test` now works with the `tzset()` warning gone.

Add more info on how to run tests

Free mrb->rescue and mrb->ensure.

Check iv in mrb_iv_remove.

[xpcat] Print usage in the parameter error.

fix out-of-bound access on compiling empty blocks.

Avoid using memset