Fix fallouts from Linux header inclusion

This is an all-in-one fix for the smaller and bigger mistakes of the
build system changes for accompanied Linux headers:
 - only enable KVM and vhost on Linux hosts
 - fix powerpc asm header symlink
 - do not use Linux headers on non-Linux hosts
 - fix kvmclock for !CONFIG_KVM
 - fix s390 build on non-Linux hosts

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Andreas Färber <andreas.faerber@web.de>
Tested-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/configure b/configure
index 856b41e..e523976 100755
--- a/configure
+++ b/configure
@@ -113,7 +113,7 @@
 curses=""
 docs=""
 fdt=""
-kvm="yes"
+kvm=""
 nptl=""
 sdl=""
 vnc="yes"
@@ -129,7 +129,7 @@
 xen_ctrl_version=""
 linux_aio=""
 attr=""
-vhost_net="yes"
+vhost_net=""
 xfs=""
 
 gprof="no"
@@ -457,6 +457,8 @@
   linux="yes"
   linux_user="yes"
   usb="linux"
+  kvm="yes"
+  vhost_net="yes"
   if [ "$cpu" = "i386" -o "$cpu" = "x86_64" ] ; then
     audio_possible_drivers="$audio_possible_drivers fmod"
   fi
@@ -3444,19 +3446,21 @@
 fi
 
 # use included Linux headers
-includes="-I\$(SRC_PATH)/linux-headers $includes"
-mkdir -p linux-headers
-case "$cpu" in
-i386|x86_64)
-  symlink $source_path/linux-headers/asm-x86 linux-headers/asm
-  ;;
-ppcemb|ppc|ppc64)
-  symlink $source_path/linux-headers/asm-x86 linux-headers/asm
-  ;;
-s390x)
-  symlink $source_path/linux-headers/asm-s390 linux-headers/asm
-  ;;
-esac
+if test "$linux" = "yes" ; then
+  includes="-I\$(SRC_PATH)/linux-headers $includes"
+  mkdir -p linux-headers
+  case "$cpu" in
+  i386|x86_64)
+    symlink $source_path/linux-headers/asm-x86 linux-headers/asm
+    ;;
+  ppcemb|ppc|ppc64)
+    symlink $source_path/linux-headers/asm-powerpc linux-headers/asm
+    ;;
+  s390x)
+    symlink $source_path/linux-headers/asm-s390 linux-headers/asm
+    ;;
+  esac
+fi
 
 echo "LDFLAGS+=$ldflags" >> $config_target_mak
 echo "QEMU_CFLAGS+=$cflags" >> $config_target_mak