patch: do not try to use std::span, because clang 8 can't

libreoffice.spec

@@ -86,6 +86,8 @@ Patch1:		libreoffice-4.2.5-icu-49.patch
 Patch2:		libreoffice-6.3.5.2-kde4-detection.patch
 # https://github.com/LibreOffice/core/commit/6fbfad6b
 Patch3:		6fbfad6b00e8c35346ee59cd32a0d7ccc0d8c19c.patch
+# For Clang <9, GCC <10, when building with libstdc++-gcc10
+Patch4:		span-old-compilers.patch
 
 # ROSA vendor patch
 Patch100:	libreoffice-4.1-vendor.patch

span-old-compilers.patch

@@ -0,0 +1,22 @@
+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>
+