Second attempt at dealing with newer clang's propensity to accept, yet warn
about, unknown warning flags.

* build/ac-macros/compiler.m4 (SVN_CC_MODE_SETUP, SVN_CXX_MODE_SETUP):
   Instead of trying to use -Wno-unknown-warning-option, add the invoerse
   -Werror=unknown-warning-option to the default C(XX)FLAGS.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481772 13f79535-47bb-0310-9956-ffa450edef68