TRAVIS-CI:  Document how this works

This adds a few words about how we're using travis-CI.
Travis-CI:  Use matrix layout

Because clang doesn't like -W$FOO directives in the CFLAGS, try and generate
jobs which reflect this, by tweaking CFLAGS to just "-g -O2" when clang is
used.
configure.ac:  Appease Clang

For one of the configuration tests, the code-path can have resulted in a
literal "return;" in main() which is invalid because the standard says that
an int should be returned.  Remove this line.

This allows Clang to continue and for ./configure not to fail its tests.
TODO:  +-
TODO:  Mention doxygen
Remove HAVE_STROKE related code blocks

When we remapped master to undo the reindentation of some parts of the code,
this got lost.  Since stroke support had already been removed by me before
that point, ensure this is reapplied.
Update passive grabs tracking modifiers

In the case of GTK's link widget which doesn't handle passive grabs
following FVWM's window bindings (which assumes the underlying application
can handle a PointerReplay), update the grabs in this context by alwas
ungrabbing the button and grabbing it for modifiers other than
ALL_MODIFIERS.

Signed-off-by: Thomas Adam <thomas@fvwm.org>