O_CLOEXEC was added in Mac OS X 10.7.

Revert to fcntl() to set the CLOEXEC flag for older system versions.

--- ./src/commonui/ipcmutex.cpp.orig	2021-03-19 14:29:09.000000000 +0100
+++ ./src/commonui/ipcmutex.cpp	2023-03-25 19:31:11.000000000 +0100
@@ -42,7 +42,8 @@
 			fz::scoped_lock lock{private_mutex};
 			fn = lockfile_path + L"lockfile";
 		}
-		m_fd = open(fz::to_native(fn).c_str(), O_CREAT | O_RDWR | O_CLOEXEC, 0644);
+		m_fd = open(fz::to_native(fn).c_str(), O_CREAT | O_RDWR, 0644);
+		fcntl(m_fd, F_SETFD, FD_CLOEXEC);
 	}
 	--m_instanceCount;
 #endif
--- ./src/engine/logging.cpp.orig	2020-06-18 16:06:46.000000000 +0200
+++ ./src/engine/logging.cpp	2023-03-25 19:26:47.000000000 +0100
@@ -108,8 +108,10 @@
 	if (m_log_fd == INVALID_HANDLE_VALUE) {
 		DWORD err = GetLastError();
 #else
-	m_log_fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT | O_CLOEXEC, 0644);
-	if (m_log_fd == -1) {
+	m_log_fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT, 0644);
+	if (m_log_fd != -1) {
+		fcntl(m_log_fd, F_SETFD, FD_CLOEXEC);
+	} else {
 		int err = errno;
 #endif
 		l.unlock(); //Avoid recursion
@@ -272,7 +274,7 @@
 			while ((rc = fcntl(m_log_fd, F_SETLKW, &lock)) == -1 && errno == EINTR);
 
 			// Ignore any other failures
-			int fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT | O_CLOEXEC, 0644);
+			int fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT, 0644);
 			if (fd == -1) {
 				int err = errno;
 
@@ -283,6 +285,7 @@
 				log(logmsg::error, _("Could not open log file: %s"), GetSystemErrorDescription(err));
 				return;
 			}
+			fcntl(fd, F_SETFD, FD_CLOEXEC);
 			struct stat buf2;
 			rc = fstat(fd, &buf2);
 
@@ -302,13 +305,14 @@
 			close(fd);
 
 			// Get the new file
-			m_log_fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT | O_CLOEXEC, 0644);
+			m_log_fd = open(m_file.c_str(), O_WRONLY | O_APPEND | O_CREAT, 0644);
 			if (m_log_fd == -1) {
 				int err = errno;
 				l.unlock(); // Avoid recursion
 				log(logmsg::error, _("Could not open log file: %s"), GetSystemErrorDescription(err));
 				return;
 			}
+			fcntl(m_log_fd, F_SETFD, FD_CLOEXEC);
 
 			if (!rc) {
 				// Rename didn't fail
--- ./src/interface/edithandler.cpp.orig	2022-04-21 05:30:03.000000000 +0200
+++ ./src/interface/edithandler.cpp	2023-03-25 19:58:15.000000000 +0100
@@ -143,8 +143,9 @@
 	std::wstring const lockfile = temp + L"fz3temp-lockfile";
 	if (wxFileName::FileExists(lockfile)) {
 #ifndef __WXMSW__
-		int fd = open(fz::to_string(lockfile).c_str(), O_RDWR | O_CLOEXEC, 0);
+		int fd = open(fz::to_string(lockfile).c_str(), O_RDWR, 0);
 		if (fd >= 0) {
+			fcntl(fd, F_SETFD, FD_CLOEXEC);
 			// Try to lock 1 byte region in the lockfile. m_type specifies the byte to lock.
 			struct flock f = {};
 			f.l_type = F_WRLCK;
@@ -234,8 +235,9 @@
 	}
 #else
 	auto file = fz::to_native(m_localDir) + "fz3temp-lockfile";
-	m_lockfile_descriptor = open(file.c_str(), O_CREAT | O_RDWR | O_CLOEXEC, 0600);
+	m_lockfile_descriptor = open(file.c_str(), O_CREAT | O_RDWR, 0600);
 	if (m_lockfile_descriptor >= 0) {
+		fcntl(m_lockfile_descriptor, F_SETFD, FD_CLOEXEC);
 		// Lock 1 byte region in the lockfile.
 		struct flock f = {};
 		f.l_type = F_WRLCK;