build: Require usage of C89

This requires the usage of the C89 standard for all source files. It
also fixes a wrong check for the compiler flags (was checking this for
CXX compiler instead of the C compiler).

1 files changed, 1 insertions, 0 deletions

diff --git a/include/ouroboros/common.h b/include/ouroboros/common.h
index 90431208..09900a1f 100644
--- a/include/ouroboros/common.h
+++ b/include/ouroboros/common.h
@@ -26,6 +26,7 @@
 #include <stdint.h>
 #include <unistd.h>
 #include <stdbool.h>
+#include <errno.h>
 
 typedef uint32_t port_id_t;