[config] do not enable rtti

We don't need it, and it is not always enabled in LLVM or clang, without
reliable way to check (`llvm-config --has-rtti` can return `YES` while clang
is built without rtti).

configure.ac

@@ -104,7 +104,7 @@ CLANG_CXX_COMMONFLAGS="$($LLVM_CONFIG --cppflags) -fPIC"
 CLANG_CFLAGS="$($LLVM_CONFIG --cppflags) -fPIC"
 CLANG_INCDIR=$($LLVM_CONFIG --includedir)
 CLANG_LINKFLAGS="$($LLVM_CONFIG --ldflags)"
-CLANG_CXXFLAGS="$($LLVM_CONFIG --cxxflags) -fPIC"
+CLANG_CXXFLAGS="$($LLVM_CONFIG --cxxflags) -fPIC -fno-rtti"
 CLANG_SYSLIBS="$($LLVM_CONFIG --system-libs)"
 CLANG_BIN_DIR="$($LLVM_CONFIG --bindir)"
 if test "$LLVM_SHARED_MODE" = "shared"; then