LLVM10 headers need C++14, so override the override.

--- meson.build
+++ meson.build
@@ -730,7 +730,7 @@ if _opencl != 'disabled'
       #error "AltiVec not enabled"
       #endif''',
       name : 'Altivec')
-    clover_cpp_std += ['cpp_std=gnu++11']
+    clover_cpp_std += ['cpp_std=gnu++14']
   endif
 else
   dep_clc = null_dep