XXX temporary fix:
Bug 1973994 - mozjs-140.pc does not contain -DXP_UNIX on Linux
https://github.com/ptomato/mozjs/commit/9aa8b4b051dd539e0fbd5e08040870b3c712a846

Index: js/src/js-config.h.in
--- js/src/js-config.h.in.orig
+++ js/src/js-config.h.in
@@ -68,4 +68,8 @@
 #undef MOZJS_MAJOR_VERSION
 #undef MOZJS_MINOR_VERSION
 
+/* Controls API in UniquePtrExtensions.h. */
+#undef XP_UNIX
+#undef XP_WIN
+
 #endif /* js_config_h */
