configure: Add system = 'linux' for meson when cross-compiling

Meson needs the "system = xyz" line when cross-compiling. We are already
adding a "system = 'windows'" for the MinGW cross-compilation case here,
so let's add a "system = 'linux'" now for Linux hosts, too.

Message-Id: <20200823111757.72002-2-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -8172,6 +8172,9 @@ if test -n "$cross_prefix"; then
             ?:*) pre_prefix=/ ;;
         esac
     fi
+    if test "$linux" = "yes" ; then
+        echo "system = 'linux'" >> $cross
+    fi
     case "$ARCH" in
         i386|x86_64)
             echo "cpu_family = 'x86'" >> $cross