ci: convert one build to cover strict C++11 usage

Without this, we do not cover compilation with only C++11 until nightly
testing, at which point many builds require it, and failures block
merging anything that was staged.

.gitlab/ci/configure_fedora31_ninja.cmake

@@ -1,3 +1,7 @@
 set(CMake_TEST_ISPC "ON" CACHE STRING "")
 set(CMake_TEST_GUI "ON" CACHE BOOL "")
+
+# Cover compilation with C++11 only and not higher standards.
+set(CMAKE_CXX_STANDARD "11" CACHE STRING "")
+
 include("${CMAKE_CURRENT_LIST_DIR}/configure_fedora31_common.cmake")