Convert cc_prefix.sh and gcc-rtags-hook.sh to POSIX sh

While here, silence shellcheck warnings.

Author: Jehops

bin/cc_prefix.sh

@@ -1,6 +1,6 @@
-#!/bin/bash
+#!/bin/sh
 
-[ -z "$RTAGS_DISABLED" ] && [ -x "$(which rc)" ] && rc --silent --compile "$@" &
+[ -z "$RTAGS_DISABLED" ] && [ -x "$(command -v rc)" ] && rc --silent --compile "$@" &
 [ "$RTAGS_RMAKE" ] && exit 1
 compiler="$1"
 shift

bin/gcc-rtags-hook.sh

@@ -1,6 +1,6 @@
-#!/bin/bash
+#!/bin/sh
 
-if [ -z "$RTAGS_DISABLED" ] && [ -x "$(which rc)" ]; then
+if [ -z "$RTAGS_DISABLED" ] && [ -x "$(command -v rc)" ]; then
     rc --silent --compile "$ICECC_CXX" "$@" &
 fi