tools/libxl: Fix build of libxl with ./configure --disable-blktap2

Autoconf already has appropriate Linux/non-Linux logic for CONFIG_BLKTAP2.

Let LIBXL_BLKTAP inherit directly from CONFIG_BLKTAP2, rather than requiring a
further LIBXL_BLKTAP=n in addition to ./configure --disable-blktap2 to get
libxl to successfully link under Linux.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/Rules.mk

@@ -55,11 +55,7 @@ CFLAGS_libxenvchan = -I$(XEN_LIBVCHAN)
 LDLIBS_libxenvchan = $(SHLIB_libxenctrl) $(SHLIB_libxenstore) -L$(XEN_LIBVCHAN) -lxenvchan
 SHLIB_libxenvchan  = -Wl,-rpath-link=$(XEN_LIBVCHAN)
 
-ifeq ($(CONFIG_Linux),y)
-LIBXL_BLKTAP ?= y
-else
-LIBXL_BLKTAP ?= n
-endif
+LIBXL_BLKTAP ?= $(CONFIG_BLKTAP2)
 
 ifeq ($(LIBXL_BLKTAP),y)
 CFLAGS_libblktapctl = -I$(XEN_BLKTAP2)/control -I$(XEN_BLKTAP2)/include $(CFLAGS_xeninclude)