cmake-gui: Improve label for default platform

Specify the default platform to be used when the platform field is empty
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

00:29:13

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:56:44

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:23:21

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

00:28:33

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

00:15:18

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

01:00:00

passed cmake-terfin-windows-release-vs2017

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager