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>

---
v2: Clarify deliberate INSTALL_SHLIB and SYMLINK_SHLIB breakage.

config/MiniOS.mk

@@ -7,3 +7,4 @@ LDFLAGS += $(DEF_LDFLAGS) $(ARCH_LDFLAGS)
 
 # Override settings for this OS
 PTHREAD_LIBS =
+nosharedlibs=y