configure: cross-compiling with empty cross_prefix

The iOS toolchain does not use the host prefix naming convention. So we
need to enable cross-compile options while allowing the PREFIX to be
blank.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Joelle van Dyne <j@getutm.app>
Message-id: 20210126012457.39046-3-j@getutm.app
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -238,6 +238,7 @@ cpu=""
 iasl="iasl"
 interp_prefix="/usr/gnemul/qemu-%M"
 static="no"
+cross_compile="no"
 cross_prefix=""
 audio_drv_list=""
 block_drv_rw_whitelist=""
@@ -469,6 +470,7 @@ for opt do
   optarg=$(expr "x$opt" : 'x[^=]*=\(.*\)')
   case "$opt" in
   --cross-prefix=*) cross_prefix="$optarg"
+                    cross_compile="yes"
   ;;
   --cc=*) CC="$optarg"
   ;;
@@ -1691,7 +1693,7 @@ $(echo Deprecated targets: $deprecated_targets_list | \
   --target-list-exclude=LIST exclude a set of targets from the default target-list
 
 Advanced options (experts only):
-  --cross-prefix=PREFIX    use PREFIX for compile tools [$cross_prefix]
+  --cross-prefix=PREFIX    use PREFIX for compile tools, PREFIX can be blank [$cross_prefix]
   --cc=CC                  use C compiler CC [$cc]
   --iasl=IASL              use ACPI compiler IASL [$iasl]
   --host-cc=CC             use C compiler CC [$host_cc] for code run at
@@ -6290,7 +6292,7 @@ if has $sdl2_config; then
 fi
 echo "strip = [$(meson_quote $strip)]" >> $cross
 echo "windres = [$(meson_quote $windres)]" >> $cross
-if test -n "$cross_prefix"; then
+if test "$cross_compile" = "yes"; then
     cross_arg="--cross-file config-meson.cross"
     echo "[host_machine]" >> $cross
     if test "$mingw32" = "yes" ; then