configure: Moved configure's detection of Xinerama to a dedicated macro
The original check was not compliant with autoconf's syntax, did not have
a very good behaviour for user and was not easy to make evolve.
The new macro:
- uses as much as possible autoconf macros for portability and code
consistency;
- checks also for header usability with the compiler;
- provides a consistent behaviour on yes/no/auto (if user explicitly
enables support, do not silently disable if not found; if library is found
but not the header, complain to let user install it or explicitly disable
support)
The feature is also no more disabled by default, as it is something user
would expect from a modern window manager.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>