Add a way to run tippecanoe single-threaded for profiling

thread.cpp

@@ -0,0 +1,22 @@
+#include <stdlib.h>
+#include "thread.hpp"
+
+// It is harder to profile tippecanoe because of its normal use of multiple threads.
+// If you set TIPPECANOE_NO_THREADS in the environment, it will run everything in
+// the main thread instead for more straightforward profiling. (The threads will
+// then still be created so they will be reaped, but they will immediately return.)
+
+static const char *no_threads = getenv("TIPPECANOE_NO_THREADS");
+
+static void *do_nothing(void *arg) {
+	return arg;
+}
+
+int thread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg) {
+	if (no_threads != NULL) {
+		void *ret = start_routine(arg);
+		return pthread_create(thread, attr, do_nothing, ret);
+	} else {
+		return pthread_create(thread, attr, start_routine, arg);
+	}
+}