Fix build on FreeBSD (re: d41c52d)

Now the build on FreeBSD was failing because the standards macros
needed to expose SA_RESTART hid a bunch of BSD stuff. OpenBSD has
_BSD_SOURCE and NetBSD has _NETBSD_SOURCE to deal with this, but
FreeBSD and derivatives have no such thing.

src/lib/libast/features/standards:
- Define the internal undocumented macro __BSD_VISIBLE to expose
  the BSD stuff on FreeBSD.

src/lib/libcmd/Mamfile:
- Fix a parallelism bug discovered on FreeBSD (which creates
  processes extremely fast). When building with e.g. -j9,
  compilation of the *.c files started before the generation of
  cmdext.h was finished, causing the first couple of compilations
  to get an empty or incomplete cmdext.h. Add the 'prev' needed to
  wait for it.

src/lib/libast/features/standards

@@ -54,8 +54,9 @@ if tst note{ BSD (Free, Net, Open, et al) }end compile{
 	}end {
 		#define _XOPEN_SOURCE	9900
 		#define _POSIX_C_SOURCE 21000101L
-		#define _BSD_SOURCE	1
-		#define _NETBSD_SOURCE	1
+		#define _BSD_SOURCE	1	/* OpenBSD needs this */
+		#define _NETBSD_SOURCE	1	/* NetBSD needs this */
+		#define __BSD_VISIBLE	1	/* FreeBSD needs this */
 	}
 elif tst note{ Darwin (macOS, Mac OS X) }end compile{
 		/*

src/lib/libcmd/Mamfile

@@ -290,6 +290,9 @@ make install virtual
 	make libcmd.a
 		loop OBJ cmdinit basename cat chgrp chmod chown cksum cmp comm cp cut dirname date expr fds fmt fold getconf head id join ln logname md5sum mkdir mkfifo mktemp mv paste pathchk pids rev rm rmdir stty sum sync tail tee tty uname uniq wc revlib wclib lib
 			make %{OBJ}.o
+				note * cmdext.h is included by cmd.h but generated after cmd.h is
+				note * declared, so we declare the dependency on it here instead.
+				prev cmdext.h
 				prev %{OBJ}.c
 				exec - %{CC} %{mam_cc_FLAGS} %{CCFLAGS} %{mam_cc_NOSTRICTALIASING} -I. -I%{INCLUDE_AST} -DERROR_CATALOG=\""libcmd"\" -DHOSTTYPE=\""%{mam_cc_HOSTTYPE}"\" -D_BLD_cmd -c %{<}
 			done