build: allow setting a custom GIT binary for transparent proxying

Some users can't run a bare 'git' command, due to need for a transparent
proxying solution such as 'tsocks'. This adds an argument to configure to
let users specify such a thing:

  ./configure --with-git="tsocks git"

The submodule script is also updated to give the user a hint about using this
flag, if we fail to checkout modules.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

configure

@@ -271,6 +271,7 @@ then
 else
     git_submodules=""
 fi
+git="git"
 
 # Don't accept a target_list environment variable.
 unset target_list
@@ -1303,6 +1304,8 @@ for opt do
   ;;
   --enable-capstone=system) capstone="system"
   ;;
+  --with-git=*) git="$optarg"
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -5413,6 +5416,7 @@ echo "local state directory   queried at runtime"
 echo "Windows SDK       $win_sdk"
 fi
 echo "Source path       $source_path"
+echo "GIT binary        $git"
 echo "GIT submodules    $git_submodules"
 echo "C compiler        $cc"
 echo "Host C compiler   $host_cc"
@@ -5604,6 +5608,7 @@ echo "extra_cxxflags=$EXTRA_CXXFLAGS" >> $config_host_mak
 echo "extra_ldflags=$EXTRA_LDFLAGS" >> $config_host_mak
 echo "qemu_localedir=$qemu_localedir" >> $config_host_mak
 echo "libs_softmmu=$libs_softmmu" >> $config_host_mak
+echo "GIT=$git" >> $config_host_mak
 echo "GIT_SUBMODULES=$git_submodules" >> $config_host_mak
 
 echo "ARCH=$ARCH" >> $config_host_mak