Unable to Cross Compile

[Uglymotha]

Currently it is not possible to cross compile gssproxy. This is due to two autoconf macros.

1. In external/dinglibs.m4 AC_RUN_IFELSE is used without cross compile action, leading to error: cannot run test program while cross compiling.
2. In external/docbook.m4 AC_CHECK_FILE is used, leading to error: cannot check for file existence when cross compiling

I propose to fix 1. by adding a cross compile action to AC_RUN_IFELSE displaying a warning message for the user to make sure libini_config supports extended INI_MS_DETECT and maybe also fix the stale URL (https://pagure.io/SSSD/ding-libs/pull-request/3172) in the current error message. Maybe the test can just be removed altogether.

2 Can be simply fixed by replacing AC_CHECK_FILE with an explicit if ! test .....

If you concur, I can prepare a PR.