Revision 1481772

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.

* configure.ac: Dump the various sets of C and C++ options, for easier debugging.