GNUInstallDirs: avoid unwanted variable dereference
In some situations, it seems that the variable
0 is defined. In the
case found, it was set to
1. This makes the detection of the missing
third argument bogus and unnecessarily triggers a warning.
This oversight was introduced in 229b5ee9 (GNUInstallDirs: Add dir argument to GNUInstallDirs_get_absolute_install_dir, 2020-10-31) prior to CMake 3.20's release cycle.
Eligible for 3.20.6 as well (as 3.20 contained !5450 (merged)).