Issue #27490: Do not build pgen when cross-compiling

The dependendency on the $(PGEN) variable must only be
set when not cross-compiling. When cross-compiling,
$(PGEN) will not be used, so no need to build it.

Patch by Thomas Perl.
diff --git a/Misc/ACKS b/Misc/ACKS
index 2b32fb8..a4a4cdc 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1120,6 +1120,7 @@
 Bo Peng
 Santiago Peresón
 George Peristerakis
+Thomas Perl
 Mathieu Perreault
 Mark Perrego
 Trevor Perrin