cc-wrapper: Pass shellcheck and other cleanups

In many cases, this involved taking @orivej's and @edolstra's recent
ld-wrapper improvements, and applying then elsewhere.

pkgs/build-support/cc-wrapper/setup-hook.sh

@@ -1,45 +1,50 @@
 addCVars () {
-    if [ -d $1/include ]; then
+    if [[ -d "$1/include" ]]; then
         export NIX_CFLAGS_COMPILE+=" ${ccIncludeFlag:--isystem} $1/include"
     fi
 
-    if [ -d $1/lib64 -a ! -L $1/lib64 ]; then
+    if [[ -d "$1/lib64" && ! -L "$1/lib64" ]]; then
         export NIX_LDFLAGS+=" -L$1/lib64"
     fi
 
-    if [ -d $1/lib ]; then
+    if [[ -d "$1/lib" ]]; then
         export NIX_LDFLAGS+=" -L$1/lib"
     fi
 
-    if test -d $1/Library/Frameworks; then
-        export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -F$1/Library/Frameworks"
+    if [[ -d "$1/Library/Frameworks" ]]; then
+        export NIX_CFLAGS_COMPILE+=" -F$1/Library/Frameworks"
     fi
 }
 
 envHooks+=(addCVars)
 
-# Note: these come *after* $out in the PATH (see setup.sh).
+# Note 1: these come *after* $out in the PATH (see setup.sh).
+# Note 2: phase separation makes this look useless to shellcheck.
 
+# shellcheck disable=SC2157
 if [ -n "@cc@" ]; then
     addToSearchPath _PATH @cc@/bin
 fi
 
+# shellcheck disable=SC2157
 if [ -n "@binutils_bin@" ]; then
     addToSearchPath _PATH @binutils_bin@/bin
 fi
 
+# shellcheck disable=SC2157
 if [ -n "@libc_bin@" ]; then
     addToSearchPath _PATH @libc_bin@/bin
 fi
 
+# shellcheck disable=SC2157
 if [ -n "@coreutils_bin@" ]; then
     addToSearchPath _PATH @coreutils_bin@/bin
 fi
 
-if [ -z "$crossConfig" ]; then
-  ENV_PREFIX=""
+if [ -z "${crossConfig:-}" ]; then
+    ENV_PREFIX=""
 else
-  ENV_PREFIX="BUILD_"
+    ENV_PREFIX="BUILD_"
 fi
 
 export NIX_${ENV_PREFIX}CC=@out@