merge revision(s) 019cbded90ade76fdb87d02bd4d444724429fc92:

	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 | 7 +++++++
	 lib/mkmf.rb  | 3 ++-
	 2 files changed, 9 insertions(+), 1 deletion(-)

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 32829e42fc..800035b096 100644
--- a/lib/mkmf.rb
+++ b/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=@)