lib: fix definition of LIBNOTMUCH_CHECK_VERSION

lib: fix definition of LIBNOTMUCH_CHECK_VERSION

Fix bug reported in id:20160606124522.g2y2eazhhrwjsa4h@flatcap.org

Although the C99 standard 6.10 is a little non-obvious on this point,
the docs for e.g. gcc are unambiguous. And indeed in practice with the
extra space, this code fails

#include <stdio.h>
#define foo (x) (x+1)

int main(int argc, char **argv){
  printf("%d\n",foo(1));
}