COMP: Support building as an extension if only Slicer_DIR is set
COMP: Support building as an extension if only Slicer_DIR is set

This change was adapted from NIRALUser/ShapePopulationViewer@ccf0042f7 and
https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide#Slicer_4.9:_Initializing_.3CprojectName.3E_BUILD_SLICER_EXTENSION_option:_Standalone_vs_Slicer_extension_build

If the extension is configured only specifying -DSlicer_DIR:PATH=/path/to/Slicer-SuperBuild/Slicer-build,
the build system will automatically turn the option ShapeVariationAnalyzer_BUILD_SLICER_EXTENSION
to ON.

For example:

[...]
-- Checking if building as a Slicer extension
-- Checking if building as a Slicer extension - yes (Slicer_DIR is SET)
[...]


otherwise if ShapeVariationAnalyzer_BUILD_SLICER_EXTENSION is set, it
displays:

[...]
-- Checking if building as a Slicer extension
-- Checking if building as a Slicer extension - yes (ShapeVariationAnalyzer_BUILD_SLICER_EXTENSION is ON)
[...]

Finally, by default, if no option is passed, it displays:

[...]
-- Checking if building as a Slicer extension
-- Checking if building as a Slicer extension - no (ShapeVariationAnalyzer_BUILD_SLICER_EXTENSION is OFF)
[...]

DCBIA-OrthoLab/ShapeVariationAnalyzer,DCBIA-OrthoLab/ShapeVariationAnalyzer,DCBIA-OrthoLab/ShapeVariationAnalyzer