Fix typo in C++ flags setup macro. No wonder -O2 crept into the C++ compiler
flags even with --disable-optimize.

* build/ac-macros/compiler.m4 (SVN_CXX_MODE_SETUP):
   It's all about CXXFLAGS, not CFLAGS. Duh.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1535597 13f79535-47bb-0310-9956-ffa450edef68