CMAKE: doctest_discover_tests() error when ADD_LABELS is not specified

[ArcEarth]

Description

Regression introduced by #464
When doctest_discover_tests(<target>) does not have ADD_LABELS ... options, test discovery breaks with the following error:

[build] CMake Error at doctest/2.4.6/lib/cmake/doctestAddTests.cmake:60 (if):
[build]   if given arguments:
[build] 
[build]     "EQUAL" "1"
[build] 
[build]   Unknown arguments specified

Steps to reproduce

Building anything with doctest_discover_tests(<target>) without extra options.

• doctest version: v2.4.6
• cmake version: v3.19.4
• Operating System: OSX 11.4
• Compiler+version: Clang 11.0.4

[jharmer95]

I have encountered this as well. Particularly annoying because I can't easily change the file for my CI/CD
Line 60 can be changed to either

if (add_labels EQUAL 1)

or

if (${add_labels})

I can prepare a PR for this fix

[onqtam]

should be fixed in dev!