Check compiler flags one at a time, otherwise one unsupported flag can
invalidate others that are supported.

* configure.ac (--enable-optimization): Issue separate checks for
   -Wno-clobbered and -flto because SVN_CFLAGS_ADD_IFELSE does
   not treat its parameter as a list of options.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1480077 13f79535-47bb-0310-9956-ffa450edef68