configure.ac: do not let llvm-config define NDEBUG in debug builds

Re-enables assertions in src/mesa.
diff --git a/configure.ac b/configure.ac
index dde13c9..6a5bb37 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1806,7 +1806,7 @@
 if test "x$enable_gallium_llvm" = xyes; then
     if test "x$LLVM_CONFIG" != xno; then
 	LLVM_VERSION=`$LLVM_CONFIG --version`
-	LLVM_CFLAGS=`$LLVM_CONFIG --cppflags`
+	LLVM_CFLAGS=`$LLVM_CONFIG --cppflags|sed 's/-DNDEBUG\>//g'`
 	LLVM_LIBS="`$LLVM_CONFIG --libs` -lstdc++"
 
 	LLVM_LDFLAGS=`$LLVM_CONFIG --ldflags`