build: Check for -pthread before using it

QNX's qcc, for example, will complain that

    cc1: error: command-line option '-pthread' is valid for the driver but not for C

1 files changed, 8 insertions, 0 deletions

diff --git a/build/flags/pthread.c b/build/flags/pthread.c
new file mode 100644
index 0000000..db09aa4
--- /dev/null
+++ b/build/flags/pthread.c
@@ -0,0 +1,8 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/// _CFLAGS += -pthread
+
+int main(void) {
+	return 0;
+}