configure: create native file with contents of $host_cc

The argument of --host-cc is not obeyed when cross compiling.  To avoid
this issue, place it in a configuration file and pass it to meson
with --native-file.

While at it, clarify that --host-cc is not obeyed anyway when _not_
cross compiling, because cc="$host_cc" is placed before --host-cc is
processed.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -288,7 +288,7 @@ static="no"
 #  ${cross_prefix}gcc (if cross-prefix specified)
 #  system compiler
 if test -z "${CC}${cross_prefix}"; then
-  cc="$host_cc"
+  cc="cc"
 else
   cc="${CC-${cross_prefix}gcc}"
 fi
@@ -927,8 +927,8 @@ Advanced options (experts only):
   -Dmesonoptname=val       passthrough option to meson unmodified
   --cross-prefix=PREFIX    use PREFIX for compile tools, PREFIX can be blank [$cross_prefix]
   --cc=CC                  use C compiler CC [$cc]
-  --host-cc=CC             use C compiler CC [$host_cc] for code run at
-                           build time
+  --host-cc=CC             when cross compiling, use C compiler CC for code run
+                           at build time [$host_cc]
   --cxx=CXX                use C++ compiler CXX [$cxx]
   --objcc=OBJCC            use Objective-C compiler OBJCC [$objcc]
   --extra-cflags=CFLAGS    append extra C compiler flags CFLAGS
@@ -1892,7 +1892,6 @@ if test "$skip_meson" = no; then
   echo "windres = [$(meson_quote $windres)]" >> $cross
   echo "windmc = [$(meson_quote $windmc)]" >> $cross
   if test "$cross_compile" = "yes"; then
-    cross_arg="--cross-file config-meson.cross"
     echo "[host_machine]" >> $cross
     echo "system = '$targetos'" >> $cross
     case "$cpu" in
@@ -1909,6 +1908,14 @@ if test "$skip_meson" = no; then
     else
         echo "endian = 'little'" >> $cross
     fi
+    cross_arg="--cross-file config-meson.cross"
+
+    native="config-meson.native.new"
+    echo "# Automatically generated by configure - do not modify" > $native
+    echo "[binaries]" >> $native
+    echo "c = [$(meson_quote $host_cc)]" >> $native
+    mv $native config-meson.native
+    cross_arg="$cross_arg --native-file config-meson.native"
   else
     cross_arg="--native-file config-meson.cross"
   fi