build system: Introduce nosharedlibs variable.

build system: Introduce nosharedlibs variable.

Introduce a new build variable "nosharedlibs".

In tools/libxc use it instead of $(stubdom).
In tools/xenstore honour it, and build static clients.

If shared libs are disabled, do not try to install or symlink them.

Set nosharedlibs when building for MiniOS or NetBSDRump.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
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v2: Clarify deliberate INSTALL_SHLIB and SYMLINK_SHLIB breakage.