Fix cross compilation

[Uglymotha]

Fixes Issue 106 (https://github.com/gssapi/gssproxy/issues/106)

modified: external/dinglibs.m4 A warning message is displayed to the user to make sure lib_ini_config supprts extenden INI_MS_DETECT, since the test cannot be run in a cross compilation scenario. modified: external/docbook.m4 Autoconf Macro AC_CHECK_FILE has been replaced by an explicit if ! test -e ..., as AC_CHECK_FILE is not available in cross compilation scenario.