Pass explicit paths for dependencies for building lxml (#18353)

* Pass explicit paths for building lxml

* Add verbose flag to confirm version

* Revert "Add verbose flag to confirm version"

This reverts commit bead3bd.

.builders/images/macos-x86_64/extra_build.sh

@@ -26,6 +26,13 @@ fi
 # Make sure IBM MQ libraries are found under /opt/mqm even when we're using the builder cache
 sudo cp -Rf "${DD_PREFIX_PATH}/mqm" /opt
 
+# lxml has some custom logic for finding the libxml and libxslt libraries that it depends on,
+# which ignores existing CFLAGS / LDFLAGS,
+# based on the xml2-config and xslt-config binaries provided by those libraries.
+# We need to override those to avoid the build from picking up the system ones.
+echo "WITH_XML2_CONFIG=${DD_PREFIX_PATH}/bin/xml2-config" >> $DD_ENV_FILE
+echo "WITH_XSLT_CONFIG=${DD_PREFIX_PATH}/bin/xslt-config" >> $DD_ENV_FILE
+
 # Empty arrays are flagged as unset when using the `-u` flag. This is the safest way to work around that
 # (see https://stackoverflow.com/a/61551944)
 pip_no_binary=${always_build[@]+"${always_build[@]}"}