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
...
```

configure.ac

@@ -4227,6 +4227,13 @@ AS_IF([test -z "$MANTYPE"], [
 ])
 AC_SUBST(MANTYPE)
 
+MKMF_VERBOSE=0
+AC_ARG_ENABLE(mkmf-verbose,
+    AS_HELP_STRING([--enable-mkmf-verbose], [enable verbose in mkmf]),
+    [MKMF_VERBOSE=1],
+    [MKMF_VERBOSE=0])
+AC_SUBST(MKMF_VERBOSE)
+
 AC_ARG_ENABLE(rubygems,
 	AS_HELP_STRING([--disable-rubygems], [disable rubygems by default]),
 	[enable_rubygems="$enableval"], [enable_rubygems=yes])

lib/mkmf.rb

@@ -1964,13 +1964,14 @@ SRC
 
   def configuration(srcdir)
     mk = []
+    CONFIG['MKMF_VERBOSE'] ||= "0"
     vpath = $VPATH.dup
     CONFIG["hdrdir"] ||= $hdrdir
     mk << %{
 SHELL = /bin/sh
 
 # V=0 quiet, V=1 verbose.  other values don't work.
-V = 0
+V = #{CONFIG['MKMF_VERBOSE']}
 V0 = $(V:0=)
 Q1 = $(V:1=)
 Q = $(Q1:0=@)