Add feature flags and missing #includes.

These are needed to compile on some strict Linuxes.

src/term_mplex.c

@@ -20,6 +20,7 @@
  *
  */
 
+#define _POSIX_C_SOURCE 2
 #include <limits.h> /* PATH_MAX */
 #include <stdio.h>  /* fgets, popen, pclose */
 #include <stdlib.h> /* malloc, realloc, free, getenv */