our libcxx isnt built with _LIBCPP_HAS_PARALLEL_ALGORITHMS

Index: Pdf4QtLibCore/sources/pdfexecutionpolicy.h
--- Pdf4QtLibCore/sources/pdfexecutionpolicy.h.orig
+++ Pdf4QtLibCore/sources/pdfexecutionpolicy.h
@@ -138,7 +138,7 @@ class PDF4QTLIBCORESHARED_EXPORT PDFExecutionPolicy (p
         }
         else
         {
-            std::for_each(std::execution::seq, first, last, f);
+            std::for_each(first, last, f);
         }
     }
 
@@ -148,7 +148,7 @@ class PDF4QTLIBCORESHARED_EXPORT PDFExecutionPolicy (p
         Q_UNUSED(scope);
 
         // We always sort by single thread
-        std::sort(std::execution::seq, first, last, f);
+        std::sort(first, last, f);
     }
 
     /// Returns number of active threads for given scope
