Fix ambiguity with C++11 user-defined literals

By adding whitespace around the macros, we make it clear that we mean to
concatenate the string literals, not use a C++11 user-defined literal.

See http://gcc.gnu.org/gcc-4.7/porting_to.html, section "User-defined
literals and whitespace."  This is necessary to compile libopkele
with C++11 enabled.