Workaround for `glibc >2.24` compilation failure
Workaround for `glibc >2.24` compilation failure

`glibc 2.24` deprecated `readdir_r` and patched `readdir` for threadsafeness.  This Makefile upgrades warnings to errors with `CWFLAGS=-Werror`, so compilation fails in the presence of `glibc >2.24`.  This is a problem in Arch today, and other distros in the near future.

The best solution may be to check the `glibc` version and replace `readdir_r` with `readdir` if `glibc >2.24`.  In the meantime, using `CWFLAGS+=-Wno-error=deprecated-declarations` in the Makefile in the case of `glibc >2.24` allows `glibc 2.24` users to compile Urbit.

Depends on `getconf` and `expr` shell built-ins, which should be available everywhere.
jfranklin9000/urbit,juped/urbit,ngzax/urbit,juped/urbit,cgyarvin/urbit-1,max19/urbit,cgyarvin/urbit-1,max19/urbit,juped/urbit,max19/urbit,ngzax/urbit,cgyarvin/urbit-1,cgyarvin/urbit-1,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,juped/urbit,cgyarvin/urbit-1,jfranklin9000/urbit,max19/urbit,jfranklin9000/urbit,max19/urbit,urbit/urbit,cgyarvin/urbit-1,ngzax/urbit,jfranklin9000/urbit,max19/urbit,ngzax/urbit,juped/urbit,cgyarvin/urbit-1,juped/urbit,juped/urbit,max19/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,cgyarvin/urbit-1,urbit/urbit,max19/urbit,juped/urbit