dev-lang/python-exec: Respect PYTHON_TARGETS (NFC)

Actually use PYTHON_TARGETS when building implementation list.  This is
NFC since all targets are package.use.forced but it lets advanced users
customize behavior via un-forcing those flags.

Bug: https://bugs.gentoo.org/693910
Signed-off-by: Michał Górny <mgorny@gentoo.org>

dev-lang/python-exec/python-exec-2.4.6-r1.ebuild

@@ -27,8 +27,10 @@ RDEPEND="
 src_configure() {
 	local pyimpls=() i EPYTHON
 	for i in "${_PYTHON_ALL_IMPLS[@]}"; do
-		python_export "${i}" EPYTHON
-		pyimpls+=( "${EPYTHON}" )
+		if use "python_targets_${i}"; then
+			python_export "${i}" EPYTHON
+			pyimpls+=( "${EPYTHON}" )
+		fi
 	done
 
 	local myconf=(