Remove compiler warnings when building Bionic.

Also add missing declarations to misc. functions.
Fix clearerr() implementation (previous was broken).
Handle feature test macros like _POSIX_C_SOURCE properly.

Change-Id: Icdc973a6b9d550a166fc2545f727ea837fe800c4
diff --git a/libc/unistd/sysconf.c b/libc/unistd/sysconf.c
index dedc5bc..27b113c 100644
--- a/libc/unistd/sysconf.c
+++ b/libc/unistd/sysconf.c
@@ -96,7 +96,7 @@
     case _SC_COLL_WEIGHTS_MAX:  return _POSIX2_COLL_WEIGHTS_MASK;
 #endif
 #ifdef _POSIX2_EXPR_NEST_MAX
-    case _SC_EXPR_NEXT_MASK:    return _POSIX2_EXPR_NEST_MAX;
+    case _SC_EXPR_NEST_MAX:    return _POSIX2_EXPR_NEST_MAX;
 #endif
 #ifdef _POSIX2_LINE_MAX
     case _SC_LINE_MAX:          return _POSIX2_LINE_MAX;