ebuild.sh: EAPI conditional usex dummy

This probably makes no difference in practice, since it only affects
the "depend" phase. Note that we're allowed to use EAPI conditionals
here, since we parse the EAPI on the python side and pass it to bash.
diff --git a/bin/ebuild.sh b/bin/ebuild.sh
index aa3dcdd..1b15bd0 100755
--- a/bin/ebuild.sh
+++ b/bin/ebuild.sh
@@ -33,7 +33,11 @@
 	done
 	# These dummy functions return false in non-strict EAPIs, in order to ensure that
 	# `use multislot` is false for the "depend" phase.
-	for x in use useq usev usex ; do
+		funcs="use useq usev"
+		if ___eapi_has_usex; then
+			funcs+=" usex"
+		fi
+		for x in ${funcs} ; do
 		eval "${x}() {
 			if ___eapi_disallows_helpers_in_global_scope; then
 				die \"\${FUNCNAME}() calls are not allowed in global scope\"
@@ -47,7 +51,7 @@
 	for x in best_version has_version portageq ; do
 		eval "${x}() { die \"\${FUNCNAME}() calls are not allowed in global scope\"; }"
 	done
-	unset x
+	unset funcs x
 fi
 
 # Don't use sandbox's BASH_ENV for new shells because it does