In rosa2016.1, we are building LibreOffice with clang 8 and libstdc++ from GCC 10.
Seems that libstdc++-gcc-10 does have the span header, but clang 8 is not capable of std::span:
"error: no member named 'span' in namespace 'std'"
(I am not sure)
include/o3tl/span.hxx tries to use system <span> when available and uses the built in implementation when it is not available.
This patch tricks the compiler to always use the built in implementation.

TODO: maybe fix libstdc++-gcc10 so that it by default reports that <span> header does not exist.

// mikhailnov, 04.04.2020

--- libreoffice-6.3.5.2/include/o3tl/span.hxx.orig2	2020-04-04 17:41:58.575172521 +0300
+++ libreoffice-6.3.5.2/include/o3tl/span.hxx	2020-04-04 18:17:09.141934433 +0300
@@ -12,7 +12,7 @@
 
 #include <sal/config.h>
 
-#if __has_include(<span>)
+#if 0
 
 #include <span>