Implement header searcher to use readline in mirb if possible.

Since using mirb without readline is stressful it should detect it automatically.
By default header searcher will search `include_paths` of compiler.
Additionally in gcc toolchain it will use result of `echo | gcc -xc -Wp,-v - -fsyntax-only 2>&1` too, by defining singleton method `header_search_paths`.(clang toolchain will follow this behaviour too.)



```C
#elif ENABLE_LINENOISE
```
is replaced with
```C
#elif defined(ENABLE_LINENOISE)
```
to use `ENABLE_LINENOISE` like `ENABLE_READLINE`.