build: Support building using clang-usan and disable-optimization

Clang Undefined Behaviour Sanitizer seems to make non-constant constant
expressions, making the compilation fail when they are used within
inline assembly if no optimization is requested.

Add -O1 to the cflags to hide the misbehaviour.

Bug-Id: 683

configure

@@ -2443,7 +2443,7 @@ case "$toolchain" in
     ;;
     clang-usan)
         cc_default="clang"
-        add_cflags  -fsanitize=undefined
+        add_cflags  -fsanitize=undefined -O1
         add_ldflags -fsanitize=undefined
     ;;
     gcc-asan)