mkmf.rb: set cppflags

* ext/mkmf.rb (configuration): set the default cppflags, which is
  referred from the default CPPFLAGS, for extension libraries.
  This fixes build failure of ext/zlib on Solaris 10.
  [Bug #14746] [ruby-dev:50539]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63398 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -1982,6 +1982,7 @@ cxxflags = #{CONFIG['cxxflags']}
 optflags = #{CONFIG['optflags']}
 debugflags = #{CONFIG['debugflags']}
 warnflags = #{$warnflags}
+cppflags = #{CONFIG['cppflags']}
 CCDLFLAGS = #{$static ? '' : CONFIG['CCDLFLAGS']}
 CFLAGS   = $(CCDLFLAGS) #$CFLAGS $(ARCH_FLAG)
 INCFLAGS = -I. #$INCFLAGS