win installer: CPack error when optional targets are missing

[tkemmer]

On Windows 10/MSVC 14, package creation fails when certain (optional) components have not been built:

CPack: Create package
CPack Error: Problem running NSIS command: "C:/Program Files (x86)/NSIS/makensis.exe" "C:/Users/admin/git/ball/build15/_CPack_Packages/win64/NSIS/project.nsi"
Please check C:/Users/admin/git/ball/build15/_CPack_Packages/win64/NSIS/NSISOutput.log for errors
CPack Error: Problem compressing the directory
CPack Error: Error when generating package: BALL
NMAKE : fatal error U1077: "echo": Rückgabe-Code "0x1"
Stop.

Log:

warning 6000: unknown variable/constant "{DocumentationHTML}" detected, ignoring (macro:Deselect_required_by_Runtime:16)
Usage: SectionGetFlags section_index $(user_var: output flags)
Error in macro Deselect_required_by_Runtime on macroline 16
Error in macro MaybeSelectionChanged on macroline 10
Error in macro SectionList on macroline 7
Error in script "C:/Users/admin/git/ball/build15/_CPack_Packages/win64/NSIS/project.nsi" on line 10312 -- aborting creation process

Indeed, DocumentationHTML does not appear as a Section in project.nsi (in contrast to DocumentationExamples) when the documentation has not been built. The same error should occur for DocumentationPDF.