xspawn: Use _Fork() if available

This completes the workaround for bfs_spawn() hanging on FreeBSD with
ASan enabled.

Link: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280318

build/flags.mk

@@ -105,6 +105,7 @@ include build/exports.mk
 
 # Conditionally-supported flags
 AUTO_FLAGS := \
+    gen/flags/bind-now.mk \
     gen/flags/deps.mk \
     gen/flags/missing-var-decls.mk
 

build/flags/bind-now.c

@@ -0,0 +1,8 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/// _LDFLAGS += -Wl,-z,now
+
+int main(void) {
+	return 0;
+}

build/has/_Fork.c

@@ -0,0 +1,8 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+#include <unistd.h>
+
+int main(void) {
+	return _Fork();
+}

build/header.mk

@@ -13,6 +13,7 @@ include build/exports.mk
 # All header fragments we generate
 HEADERS := \
     gen/has/--st-birthtim.h \
+    gen/has/_Fork.h \
     gen/has/acl-get-entry.h \
     gen/has/acl-get-file.h \
     gen/has/acl-get-tag-type.h \

src/xspawn.c

@@ -612,7 +612,11 @@ static pid_t bfs_fork_spawn(struct bfs_resolver *res, const struct bfs_spawn *ct
 		goto fail;
 	}
 
+#if BFS_HAS__FORK
+	pid_t pid = _Fork();
+#else
 	pid_t pid = fork();
+#endif
 	if (pid == 0) {
 		// Child
 		bfs_spawn_exec(res, ctx, argv, envp, pipefd);