mkmf: Add a configure option to set verbose mode (V=1 or 0) in mkmf.rb.

Note this change is only for `configure.ac`, not for Windows using
`win32/configure.bat`.

```
$ ./configure --help | grep mkmf
  --enable-mkmf-verbose   enable verbose in mkmf
```

Run the following command to enable the mkmf verbose mode.

```
$ ./configure --enable-mkmf-verbose
$ grep MKMF_VERBOSE config.status
S["MKMF_VERBOSE"]="1"
```

In this mkmf verbose mode, when compiling a native extension, the
`rake compile` prints the compiling commands such as
"gcc -I. <...> path/to/file" instead of "compiling path/to/file".

```
$ git clone https://github.com/deivid-rodriguez/byebug.git
$ cd byebug
$ bundle install --standalone
$ bundle exec rake compile
...
gcc -I. <...> path/to/file
...
```

0 files changed, 0 insertions, 0 deletions