add default for --debug-llvm option

Author: nikomatsakis

configure

@@ -253,7 +253,7 @@ opt docs     1 "build documentation"
 opt optimize 1 "build optimized rust code"
 opt mingw-cross 0 "cross-compile for win32 using mingw"
 opt clang 0 "prefer gcc to clang for building the runtime"
-opt debug-llvm "build LLVM in debug mode"
+opt debug-llvm 0 "build LLVM in debug mode"
 valopt prefix "/usr/local" "set installation prefix"
 valopt llvm-root "" "set LLVM root"
 valopt target-triples "" "LLVM target triples (defaults to host if unset)"