Change ARCH_IS_WINDOWS to return false if in full cygwin install

lib/system.g

@@ -517,13 +517,20 @@ end );
 ## <Func Name="ARCH_IS_WINDOWS" Arg=''/>
 ##
 ## <Description>
-## tests whether &GAP; is running on a Windows system in Cygwin.
+## tests whether &GAP; is running on a Windows system without
+## standard POSIX tools available (such as a shell).
 ## </Description>
 ## </ManSection>
 ## <#/GAPDoc>
 ##
 BIND_GLOBAL("ARCH_IS_WINDOWS",function()
- return POSITION_SUBSTRING (GAPInfo.Architecture, "cygwin", 0) <> fail;
+ # Exit early is we are not in Cygwin
+ if POSITION_SUBSTRING (GAPInfo.Architecture, "cygwin", 0) = fail then
+ return false;
+ fi;
+ # Check if programs we need exist in their standard Cygwin locations
+ return not IsExistingFile("/usr/bin/sh") or
+ not IsExistingFile("/usr/bin/xdg-open");
 end);
 
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