Also fix #3499.  The issue was solved by #3462.

Avoid infinity loop when empty string pattern

\1 sequences as empty strings

Terminate loop if the value is longer than the last otherwise.

Check modifiable for String `bang' methods

String#index shouldn't return nil when "".index ""

Instead of shifting mruby VM stack, we always use CALL_MAXARGS.

There are 3 levels of C++ exception handling:
* default - no C++ exception (use setjmp/longjmp)
* enable_cxx_exception (use C++ exceptions with C ABI)
* enable_cxx_abi (use C++ ABI including exceptions)

ref #3267 #3470

By this commit, mruby do not use C++ ABI mode unless
you specify explicitly. It compiles C files by C
compilers, with C++ exception enabled when it sees
C++ files in your configured mrbgems.

I haven't tried visualcpp, so please submit an issue
if you see any problem with C++ gems on Windows.