maint: make spacing around "=" consistent, even in IF_LINT

E.g.,
  -  size_t desired_width IF_LINT (= 0);
  +  size_t desired_width IF_LINT ( = 0);
Use this command:
  g grep -l IF_LINT | grep '\.[ch]$' \
    | xargs perl -pi -e 's/(IF_LINT \()= /$1 = /'

src/expr.c

@@ -547,7 +547,7 @@ trace (fxn)
 static VALUE *
 docolon (VALUE *sv, VALUE *pv)
 {
-  VALUE *v IF_LINT (= NULL);
+  VALUE *v IF_LINT ( = NULL);
   const char *errmsg;
   struct re_pattern_buffer re_buffer;
   char fastmap[UCHAR_MAX + 1];