regex.c: use of xmalloc/xrealloc breaks lib-src build

[Tim Van Holder]

The changes of 20060219 to regex.c, which introduced direct calls to
xmalloc and xrealloc, break lib-src builds (etags/ctags in particular).
This is because those two functions are only available in emacs context
(then regex.c includes lisp.h and malloc/realloc are mapped to
xmalloc/xrealloc using #defines).
Revisions 1.206-1.208 of regex.c should be rolled back to make building
etags/ctags work again.