* configure.in: improve ICC (Intel C Compiler) support.

	* configure.in (CXX): The name of icc's c++ compiler is `icpc`.

	* configure.in (warnings): Add `-diag-disable=2259` to suppress
	  noisy warnings: "non-pointer conversion from "..." to "..." may
	  lose significant bits".

	* configure.in (optflags): Add `-fp-model precise` like -fno-fast-math.

	* lib/mkmf.rb: icc supports -Werror=division-by-zero
	  and -Werror=deprecated-declarations, but doesn't support
	  -Wdivision-by-zero and -Wdeprecated-declarations.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@58143 b2dd03c8-39d4-4d8f-98ff-823fe69b080e