Make execinfo.h optional (fixes musl libc build)

finally got the chance to build on Alpine Linux, which uses musl libc instead of glibc. while it was building fine for the most part, there was one place where we used execinfo.h, a non-standard header that most UNIX-systems (but not all, as demonstrated here!) support. fortunately, the header is not necessary to allow SRB2 to run, and can be safely made optional using preprocessor directives.

so, to build using musl libc, the environment variable NOEXECINFO=1 must be set using this patch. this was the easiest way to solve this and it does the job, but we're going to have to document this later on the wiki.