Along with this change, compat code has been added (from NetBSD)
for systems that don't have it. The compat code has been reorganized
to be in a common place and its prototypes in compat.h.
The configure scripts checks if strcasestr() is available, and
uses compat code if not found.
This fixes issue #2 on github.com/vanilla/xbps.
Changes included in this set:
* Added strlcat() and strlcpy() from OpenBSD, always use them if the
system does not have them built in.
* Changed an array of PATH_MAX size allocated in the stack, to a dynamically
allocated buffer from heap. This should reduce memory usage a bit.
* Simplify code that implemented a homegrown realpath(3) implementation,
simply use realpath(3).
* If compiler supports -fstack-protector, build all code with
-D_FORTIFY_SOURCE=2 and --param ssp-buffer-size=1 so that all
buffers are protected.
