include: explicitly define environ for BSDs

In file included from interop/PassItem.cpp:11:
include/subprocess.h:821:90: error: use of undeclared identifier 'environ'
        if (int err{::posix_spawnp(&pid, sh.argv()[0], action.get(), nullptr, sh.argv(), environ)}; err != 0)
                                                                                         ^

See also https://git.musl-libc.org/cgit/musl/commit/?id=d200bd727bea
https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_02_02 says:
> All identifiers in this volume of POSIX.1-2017, except `environ`,
> are defined in at least one of the headers, as shown in XBD Headers.

include/subprocess.h

@@ -55,6 +55,9 @@ extern "C"
 #include <sys/wait.h>
 #include <unistd.h>
 #include <wordexp.h>
+
+/* Have to be manually defined according to POSIX */
+extern char **environ;
 }
 
 namespace subprocess