Fix CFLAGS and CXXFLAGS in RbConfig::CONFIG on FreeBSD and NetBSD/amd64.

* configure.in: Fix a bug introduced in r38342 that the cflagspat
  substitution is messed up by the way CFLAGS and optflags are
  modified, which affected FreeBSD and NetBSD/amd64 when
  configured to use libexecinfo.  This bug resulted in CFLAGS and
  CXXFLAGS in RbConfig::CONFIG having warnflags expanded in them,
  forcing third-party C/C++ extensions to follow what warnflags
  demands, like ANSI/ISO-C90 conformance.  ref [Bug #7101]




git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e