Check*CompilerFlag: Document use of CMAKE_REQUIRED_DEFINITIONS (#14309)

Explain how CMAKE_REQUIRED_DEFINITIONS is set before calling the
check_*_source_compiles macros.
Genex: Allow relative paths in INSTALL_INTERFACE.

These paths can be prepended with the ${_IMPORT_PREFIX} generated
in the export file.

Such relative paths were previously an error.
Merge topic 'doc-CMAKE_lang_FLAGS'

41bb95f Document CMAKE_<LANG>_FLAGS variable (#14305)
Merge topic 'fix-FindQt4-available-modules'

d57c1c1 FindQt4: Re-add QAxServer to the QT_MODULES.
Merge topic 'compile-defs-debugging'

a43e5e0 Test COMPILE_DEFINITIONS target property get/set/get round-trip
Merge topic 'tll-new-signatures'

b655865 target_link_libraries: Add PUBLIC/PRIVATE/INTERFACE keyword signature
Tests/RunCMake: Document stripping of expected output

Document that trailing newlines are stripped from the -stdout.txt
and -stderr.txt content before using as a regular expression.
export: Error when exporting a target without a language

First, it prevents a NULL dereference and second it reiterates that
targets without languages are not supported by CMake.

Add a RunCMake.ExportWithoutLanguage test exporting a library without a
languages.
Merge topic 'doc-RunCMake-strips-expected'

c80a4f9 Tests/RunCMake: Document stripping of expected output
Merge topic 'dev/export-target-without-language'

5837f19 export: Error when exporting a target without a language
cmTarget: Fix property name typo in docs.
sha2: Avoid type-punned pointer dereference (#14314)

GCC warns:

 dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

on constructs like

 *(sha_word64*)&var =

so use memcpy to perform such assignments instead.
Docs: Document file(GENERATE) CONDITION as optional.
CMake Nightly Date Stamp
Qt4Macros: Remove unneeded generate CONDITION.
Qt4Macros: Remove undefined varible use.

This may have been added in anticipation of processing COMPILE_OPTIONS,
but as moc does not accept them, there is no need for it.
Qt4Macros: Simplify some variable population.
Docs: Document existing target property debugging options.
Docs: Trim trailing whitespace in generated doc.
Docs: Generalize and de-duplicate VISIBILITY_PREFIX docs.