InstallRequiredSystemLibraries: Allow repeat use per-config

To be able to include InstallRequiredSystemLibraries more than once
(e.g. to get the Debug and Release libraries separately), clear the
internal library list for non-matching configuration.
Merge topic 'fix-valgrind-in-path'

58cc3c22 Fix ctest to allow valgrind to show up in the path name of a memcheck tool.
Help: Create proper identifiers for keywords in QtHelp.

This is necessary in order for the QHelpEngineCore::linksForIdentifier API
to work.

 http://doc-snapshot.qt-project.org/qt5-5.3/qhelpenginecore.html#linksForIdentifier

That API is used by QtCreator to enable contextual links to help files.
Help: Add context to titles in QtHelp.

This allows disambiguation of identifiers in Qt Assistant and Creator.
Encoding: Fix debug asserts parsing command line options with non-ascii chars.

With MSVC, isspace() will assert with non-ascii characters.
FindOpenCL: Added missing ENV for each environment variable reference

Also add an 'include' path suffix that may appear after the value of
some of the environment variables.
Merge topic 'InstallRequiredSystemLibraries-repeat'

15a19c31 InstallRequiredSystemLibraries: Allow repeat use per-config
Merge topic 'encoding-parse-commandline'

b6b493a4 Encoding: Fix debug asserts parsing command line options with non-ascii chars.
9dc88ee9 Merge topic 'fix-valgrind-in-path'
Merge topic 'qthelp-context-fixes'

d107949d Help: Add context to titles in QtHelp.
b5002631 Help: Create proper identifiers for keywords in QtHelp.
Merge topic 'FindOpenCL-fix-ENV'

d4295539 FindOpenCL: Added missing ENV for each environment variable reference