build-sys: do not include Windows SLIRP dependencies in $LIBS

When including the internal SLIRP library, we should add all the libraries that
it needs for the build.  Right now they are all included by QEMU, but -liphlpapi
is not needed without slirp.  Move it from LIBS to slirp_libs.

Based on a patch by Marc-André Lureau.

Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -925,7 +925,7 @@ if test "$mingw32" = "yes" ; then
   DSOSUF=".dll"
   # MinGW needs -mthreads for TLS and macro _MT.
   QEMU_CFLAGS="-mthreads $QEMU_CFLAGS"
-  LIBS="-lwinmm -lws2_32 -liphlpapi $LIBS"
+  LIBS="-lwinmm -lws2_32 $LIBS"
   write_c_skeleton;
   if compile_prog "" "-liberty" ; then
     LIBS="-liberty $LIBS"
@@ -6027,6 +6027,9 @@ case "$slirp" in
     mkdir -p slirp
     slirp_cflags="-I\$(SRC_PATH)/slirp/src -I\$(BUILD_DIR)/slirp/src"
     slirp_libs="-L\$(BUILD_DIR)/slirp -lslirp"
+    if test "$mingw32" = "yes" ; then
+      slirp_libs="$slirp_libs -lws2_32 -liphlpapi"
+    fi
     ;;
 
   system)