qt5-N: move xcrun test to post-patch block

otherwise it errors out when being parsed by a linux system.
see: https://trac.macports.org/ticket/66171

if this works properly as I think it will, I'll move this
everywhere else it is needed too.

aqua/qt5/Portfile

@@ -1177,15 +1177,17 @@ foreach {module module_info} [array get modules] {
             #
             # avoid --show-sdk-path since it is not available on all platforms
             # see https://github.com/macports/macports-ports/commit/9887e90d69f4265f9056cddc45e41551d7400235#commitcomment-49824261
-            if {[catch {exec -ignorestderr /usr/bin/xcrun --sdk macosx${configure.sdk_version} --find ld  > /dev/null 2>@1}]} {
-
-                # if no specific sdk can be found, check for a generic macosx sdk
-                if {[catch {exec -ignorestderr /usr/bin/xcrun --sdk macosx --find ld > /dev/null 2>@1}]} {
-                    ui_error "${subport}: no usable SDK can be found"
-                    return -code error "no usable SDK can be found"
-                } else {
-                    ui_debug "${subport}: using generic macosx SDK as macosx${configure.sdk_version} does not exist"
-                    configure.sdk_version
+            post-patch {
+                if {[catch {exec -ignorestderr /usr/bin/xcrun --sdk macosx${configure.sdk_version} --find ld  > /dev/null 2>@1}]} {
+
+                    # if no specific sdk can be found, check for a generic macosx sdk
+                    if {[catch {exec -ignorestderr /usr/bin/xcrun --sdk macosx --find ld > /dev/null 2>@1}]} {
+                        ui_error "${subport}: no usable SDK can be found"
+                        return -code error "no usable SDK can be found"
+                    } else {
+                        ui_debug "${subport}: using generic macosx SDK as macosx${configure.sdk_version} does not exist"
+                        configure.sdk_version
+                    }
                 }
             }