Fix specifying XCode SDK for distutils; Revert to preferring XCode Zlib (#3316)

* Fix specifying XCode SDK for distutils

* Revert to preferring XCode Zlib

It turns out, everything since 2.7.12 and 3.5.2
can use libs from modern XCode SDK
which is always available unless we're using an alternate compiler

plugins/python-build/bin/python-build

@@ -863,7 +863,9 @@ build_package_standard_build() {
       use_homebrew_readline || true
       use_homebrew_ncurses || true
       if is_mac -ge 1014; then
-        use_homebrew_zlib || use_xcode_sdk_zlib || true
+        # While XCode SDK is "always available",
+        # still need a fallback in case we are using an alternate compiler
+        use_xcode_sdk_zlib || use_homebrew_zlib || true
       else
         use_homebrew_zlib || true
       fi
@@ -873,7 +875,9 @@ build_package_standard_build() {
       use_macports_readline || true
       use_macports_ncurses || true
       if is_mac -ge 1014; then
-        use_macports_zlib || use_xcode_sdk_zlib || true
+        # While XCode SDK is "always available",
+        # still need a fallback in case we are using an alternate compiler
+        use_xcode_sdk_zlib || use_homebrew_zlib || true
       else
         use_macports_zlib || true
       fi
@@ -1593,6 +1597,7 @@ has_broken_mac_readline() {
   if can_use_macports; then
     use_macports_readline && return 1
   fi
+  return 0
 }
 
 use_homebrew_readline() {
@@ -1682,6 +1687,7 @@ has_broken_mac_openssl() {
       use_macports_openssl && return 1
     fi
   fi
+  return 0
 }
 
 use_homebrew_openssl() {
@@ -1823,6 +1829,7 @@ use_homebrew_zlib() {
     export CPPFLAGS="-I${brew_zlib}/include${CPPFLAGS:+ ${CPPFLAGS}}"
     export LDFLAGS="-L${brew_zlib}/lib${LDFLAGS:+ ${LDFLAGS}}"
     lock_in homebrew
+    rc=0
   fi
 }
 
@@ -1838,12 +1845,13 @@ use_xcode_sdk_zlib() {
   echo "python-build: use zlib from xcode sdk"
   # Since 3.9.1 (bpo-41116), CPython's setup.py logic can search default SDK's sysroot itself
   # so we don't need to do anything
-  # Since 3.5.2 (Issue #25136), distutils knows to look for Apple XCode 7+ stub libraries (.tbd)
+  # Since 2.7.12 and 3.5.2 (Issue #25136), distutils knows to look for Apple XCode 7+ stub libraries (.tbd)
   # when searching in the SDK
   # Since 2.7.4 and 3.2.1 (issue #7724), setup.py and distutils can search in the MacOS SDK when
   # it's explicitly specified with -isysroot
   
-  export CPPFLAGS="${CPPFLAGS:+$CPPFLAGS }-isysroot ${sdkroot}"
+  #distutils.unixcompiler and setup.py specifically search CFLAGS for "-isysroot"
+  export CFLAGS="${CFLAGS:+$CFLAGS }-isysroot ${sdkroot}"
   return 0
 }