diff -up thunderbird-78.3.1/comm/third_party/botan/src/lib/utils/os_utils.cpp.rhel6-clock_gettime thunderbird-78.3.1/comm/third_party/botan/src/lib/utils/os_utils.cpp
--- thunderbird-78.3.1/comm/third_party/botan/src/lib/utils/os_utils.cpp.rhel6-clock_gettime	2020-09-29 21:41:52.180409804 +0200
+++ thunderbird-78.3.1/comm/third_party/botan/src/lib/utils/os_utils.cpp	2020-09-29 21:42:54.438565760 +0200
@@ -254,6 +254,8 @@ uint64_t OS::get_high_resolution_clock()
    fall back to std::chrono.
    */
 
+#undef BOTAN_TARGET_OS_HAS_CLOCK_GETTIME
+
 #if defined(BOTAN_TARGET_OS_HAS_CLOCK_GETTIME)
 
    // The ordering here is somewhat arbitrary...