Document use of CC and CFLAGS in more detail (bug 20980, bug 21234).

Document use of CC and CFLAGS in more detail (bug 20980, bug 21234).

There are some bug reports from people setting CFLAGS not including a
-O option and then being confused when the build fails.  This patch
addresses this by documenting the proper use of CC and CFLAGS in more
detail - saying what options should go where and specifying the
requirement to compile with optimization.

The previous text incorrectly used @var markup with CC and CFLAGS.
The correct markup for environment variables is @env, but it's also
the case that passing such variables explicitly on the configure
command line is preferred to passing them in the environment, so this
patch changes the documentation to describe passing them on the
command line (and uses @code).

In many cases putting options in the wrong place may in fact work, but
I believe what I've specified is the correct rule for which options to
put where.

	[BZ #20980]
	[BZ #21234]
	* manual/install.texi (Configuring and compiling): Describe
	passing CC and CFLAGS on configure command line, not as
	environment variables.  Use @code markup on those variables.
	Specify what options go in CC and what go in CFLAGS.  Note the
	requirement to compile with optimization.
	* INSTALL: Regenerated.