Revert "Add feature flags and missing #includes."

This reverts commit dd5fa236ae.

Also, set `-std=gnu99` in Bazel build.

src/term_mplex.c

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