Enable mozjs detection.
Backported from https://github.com/libproxy/libproxy/commit/66b30ef388a45f66f9b118d51cc76b53fdb8bbf4
Bug: https://bugs.gentoo.org/811858

--- a/libproxy/cmake/modules.cmk
+++ b/libproxy/cmake/modules.cmk
@@ -44,7 +44,7 @@ if(NM_FOUND)
   set(NM_BUILD 1)
 endif(NM_FOUND)
 px_module(network_networkmanager   "${NM_BUILD}"     0       ${NM_LIBRARIES})
-px_module(pacrunner_mozjs          0                 ${BIPR} ${MOZJS_LIBRARIES})
+px_module(pacrunner_mozjs          "${MOZJS_FOUND}"  ${BIPR} ${MOZJS_LIBRARIES})
 px_module(pacrunner_natus          "${NATUS_FOUND}"  ${BIPR} ${NATUS_LIBRARIES})
 px_module(pacrunner_webkit         "${WEBKIT_FOUND}" ${BIPR} ${WEBKIT_LIBRARIES})
 px_module(wpad_dns_alias           1                 1)